Explanation
The field4 is defined incorrectly in the ABAP code snippet. The reason is that the data type c (character) cannot have a decimal places specification. The decimal places specification is only valid for the data types p (packed number) and f (floating point number)1. Therefore, the field4 definition should either omit the decimal places specification or change the data type to p or f.
References: 1: Data Types and Data Objects - ABAP Keyword Documentation - SAP Online Help

Explanation
A projection view is a RESTful Application Programming object that contains only the fields required for a particular app. A projection view is a CDS view entity that defines a projection on an existing CDS view entity or CDS DDIC-based view. A projection view exposes a subset of the elements of the projected entity, which are relevant for a specific business service. A projection view can also define aliases, virtual elements, and annotations for the projected elements. A projection view is the top-most layer of a CDS data model and prepares data for a particular use case. A projection view can have different provider contracts depending on the type of service it supports, such as transactional query, analytical query, or transactional interface.
A database view is a CDS DDIC-based view that defines a join or union of database tables. A database view has an SQL view attached and can be accessed by Open SQL or native SQL. A database view can be used as a projected entity for a projection view, but it does not contain only the fields required for a particular app.
A metadata extension is a RESTful Application Programming object that defines additional annotations for a CDS view entity or a projection view. A metadata extension can be used to enhance the metadata of a CDS data model without changing the original definition. A metadata extension does not contain any fields, but only annotations.
A data model view is a CDS view entity that defines a data model based on database tables or other CDS view entities. A data model view can have associations, aggregations, filters, parameters, and annotations. A data model view can be used as a projected entity for a projection view, but it does not contain only the fields required for a particular app.
References: CDS Projection Views - ABAP Keyword Documentation, CDS Projection Views in ABAP CDS:
What's Your Flavor, Business Object Projection - ABAP Keyword Documentation

Explanation
The access to internal tables can be optimized by using the appropriate table type and specifying the table key.
The table key is a set of fields that uniquely identifies a row in the table and determines the sorting order of the table. The table key can be either the primary key or a secondary key. The primary key is defined by the table type and the table definition, while the secondary key is defined by the user using the KEY statement1.
The following results in faster access to internal tables:
B). In a sorted internal table, specifying the primary key completely. A sorted internal table is a table type that maintains a predefined sorting order, which is defined by the primary key in the table definition. The primary key can be either unique or non-unique. A sorted internal table can be accessed using the primary key or the table index. The access using the primary key is faster than the access using the table index, because the system can use a binary search algorithm to find the row. However, the primary key must be specified completely, meaning that all the fields of the primary key must be given in the correct order and without gaps2.
D). In a hashed internal table, specifying the primary key partially from the left without gaps. A hashed internal table is a table type that does not have a predefined sorting order, but uses a hash algorithm to store and access the rows. The primary key of a hashed internal table must be unique and cannot be changed. A hashed internal table can only be accessed using the primary key, not the table index. The access using the primary key is very fast, because the system can directly calculate the position of the row using the hash algorithm. The primary key can be specified partially from the left without gaps, meaning that some of the fields of the primary key can be omitted, as long as they are the rightmost fields and there are no gaps between the specified fields.
E). In a hashed internal table, specifying the primary key completely. A hashed internal table is a table type that does not have a predefined sorting order, but uses a hash algorithm to store and access the rows. The primary key of a hashed internal table must be unique and cannot be changed. A hashed internal table can only be accessed using the primary key, not the table index. The access using the primary key is very fast, because the system can directly calculate the position of the row using the hash algorithm. The primary key can be specified completely, meaning that all the fields of the primary key must be given in the correct order.
The following do not result in faster access to internal tables, because:
A). In a sorted internal table, specifying the primary key partially from the left without gaps. A sorted internal table is a table type that maintains a predefined sorting order, which is defined by the primary key in the table definition. The primary key can be either unique or non-unique. A sorted internal table can be accessed using the primary key or the table index. The access using the primary key is faster than the access using the table index, because the system can use a binary search algorithm to find the row.
However, the primary key must be specified completely, meaning that all the fields of the primary key must be given in the correct order and without gaps. If the primary key is specified partially from the left without gaps, the system cannot use the binary search algorithm and has to perform a linear search, which is slower2.
C). In a standard internal table, specifying the primary key partially from the left without gaps. A standard internal table is a table type that does not have a predefined sorting order, but uses a sequential storage and access of the rows. The primary key of a standard internal table is the standard key, which consists of all the fields of the table row in the order in which they are defined. A standard internal table can be accessed using the primary key or the table index. The access using the primary key is slower than the access using the table index, because the system has to perform a linear search to find the row.
The primary key can be specified partially from the left without gaps, but this does not improve the access speed, because the system still has to perform a linear search.
References: 1: Internal Tables - Overview - ABAP Keyword Documentation 2: Sorted Tables - ABAP Keyword Documentation : Hashed Tables - ABAP Keyword Documentation : Standard Tables - ABAP Keyword Documentation

Explanation
The statement that can be used to expose the field "connid" of the data source "demo_ods_assoc_spfli" in the element list is A. demo_ods_assoc_spfli.connid,. This statement uses the dot notation to access the field
"connid" of the data source "demo_ods_assoc_spfli", which is an association defined on line #4. The association "demo_ods_assoc_spfli" links the data source "demo_ods" with the table "spfli" using the field
"carrid". The statement also ends with a comma to separate it from the next element in the list12.
You cannot do any of the following:
B: demo_ods_assoc_spfli-connid/: This statement uses the wrong syntax to access the field "connid" of the data source "demo_ods_assoc_spfli". The dash notation is used to access the components of a structure or a table, not the fields of a data source. The statement also ends with a slash, which is not a valid separator for the element list12.
C: spfli-connid,: This statement uses the wrong data source name to access the field "connid". The data source name should be "demo_ods_assoc_spfli",not "spfli". The statement also uses the wrong syntax to access the field "connid", as explained above12.
D:_spfli.connid/: This statement uses the wrong data source name and the wrong separator to access the field "connid". The data source name should be "demo_ods_assoc_spfli", not "_spfli". The statement also ends with a slash, which is not a valid separator for the element list12.
References: 1: ABAP CDS - SELECT, select_list - ABAP Keyword Documentation - SAP Online Help 2: ABAP CDS - SELECT, from - ABAP Keyword Documentation - SAP Online Help

Explanation
Using ABAP SQL, the select statement that selects the mat field on line #17 is:
SELECT mat FROM demo_sales_cds_so_i_ve...
This statement selects the mat field from the CDS view demo_sales_cds_so_i_ve, which is defined on line #1.
The CDS view demo_sales_cds_so_i_ve is a projection view that projects the fields of the CDS view demo_sales_cds_so_i, which is defined on line #2. The CDS view demo_sales_cds_so_i is a join view that joins the fields of the database table demo_sales_so_i, which is defined on line #3, and the CDS view demo_sales_cds_material_ve, which is defined on line #4. The CDS view demo_sales_cds_material_ve is a value help view that provides value help for the material field of the database table demo_sales_so_i. The mat field is an alias for the material field of the database table demo_sales_so_i, which is defined on line #91.
The other options are not valid because:
A). SELECT mat FROM Material... is not valid because Material is not a valid data source in the given code. There is no CDS view or database table named Material.
C). SELECT mat FROM demo_sales_so_i... is not valid because demo_sales_so_i is not a valid data source in the given code. There is no CDS view named demo_sales_so_i, only a database table. To access a database table, the keyword TABLE must be used, such as SELECT mat FROM TABLE demo_sales_so_i...
D). SELECT mat FROM demo sales cds material ve... is not valid because demo sales cds material ve is not a valid data source in the given code. There is no CDS view or database table named demo sales cds material ve. The correct name of the CDS view is demo_sales_cds_material_ve, with underscores instead of spaces.
References: 1: Projection Views - ABAP Keyword Documentation