The data declarations that will always work without truncation or rounding for the assignment gv_target = gv_source are B and C. This is because the target data type string is a variable-length character type that can hold any character string, including those of data types c (fixed-length character) and d (date). The assignment of a character or date value to a string variable will not cause any loss of information or precision, as the string variable will adjust its length to match the source value12.
You cannot do any of the following:
A . DATA gv_source TYPE string, to DATA gv_target TYPE c.: This data declaration may cause truncation for the assignment gv_target = gv_source. This is because the target data type c is a fixed-length character type that has a predefined length. If the source value of type string is longer than the target length of type c, the source value will be truncated on the right to fit the target length12.
D . DATA gv_source TYPE p LENGTH 8 DECIMALS 3. to DATA gv_target TYPE p LENGTH 16 DECIMALS 2.: This data declaration may cause rounding for the assignment gv_target = gv_source. This is because the target data type p is a packed decimal type that has a predefined length and number of decimal places. If the source value of type p has more decimal places than the target type p, the source value will be rounded to the target number of decimal places12.

The following are the explanations for each action:
A: Renaming a field in a structure that is included in the table definition causes an indirect change to the database table, as the field name in the table is derived from the structure. This change requires a table conversion, as the existing data in the table must be copied to a new table with the new field name, and the old table must be deleted.
B: Changing the field labels of a data element that is used in the table definition does not cause an indirect change to the database table, as the field labels are only used for documentation and display purposes. This change does not require a table conversion, as the existing data in the table is not affected by the change.
C: Deleting a field from a structure that is included in the table definition causes an indirect change to the database table, as the field is removed from the table as well. This change requires a table conversion, as the existing data in the table must be copied to a new table without the deleted field, and the old table must be deleted.
D: Shortening the length of a domain used in a data element that is used in the table definition causes an indirect change to the database table, as the field length in the table is derived from the domain. This change requires a table conversion, as the existing data in the table must be checked for compatibility with the new field length, and any data that exceeds the new length must be truncated or rejected.

A CDS view is a data definition that defines a data structure and a data selection from one or more data sources. A CDS view consists of several parts, but two of them are:
Extension: An extension is an optional clause that allows a CDS view to extend another CDS view by adding new elements, annotations, or associations. The extension clause has the syntax EXTEND VIEW view_name WITH view_name. The first view_name is the name of the CDS view that is being extended, and the second view_name is the name of the CDS view that is doing the extension1.
Field list: A field list is a mandatory clause that specifies the elements of the CDS view. The field list has the syntax SELECT FROM data_source { element_list }. The data_source is the name of the data source that the CDS view selects data from, and the element_list is a comma-separated list of elements that the CDS view exposes. The elements can be fields of the data source, expressions, associations, or annotations2.
The following example shows a CDS view that extends another CDS view and defines a field list:
@AbapCatalog.sqlViewName: 'ZCDS_EXT' define view Z_CDS_Extension extend view Z_CDS_Base with Z_CDS_Extension as select from ztable { // field list key ztable.id as ID, ztable.name as Name, ztable.age as Age, // extension @Semantics.currencyCode: true ztable.currency as Currency } The other options are not parts of a CDS view, but rather related concepts:
Partitioning attributes: Partitioning attributes are attributes that are used to partition a table into smaller subsets of data. Partitioning attributes are defined in the ABAP Dictionary for transparent tables and can improve the performance and scalability of data access. Partitioning attributes are not part of the CDS view definition, but rather the underlying table definition3.
Semantic table attributes: Semantic table attributes are attributes that provide additional information about the meaning and usage of a table. Semantic table attributes are defined in the ABAP Dictionary for transparent tables and can be used to enhance the data modeling and consumption of the table. Semantic table attributes are not part of the CDS view definition, but rather the underlying table definition4.