Explanation
In ABAP SQL, the syntax to retrieve the association field of a CDS view is to use the @ sign followed by the association name and the field name, separated by a period sign (.). For example, to retrieve the association field _Airline-Name of a CDS view, the syntax is @_Airline.Name. This syntax allows the access to the fields of the target data source of the association without explicitly joining the data sources1. The other options are incorrect because they use the wrong symbols or formats to access the association field.
References: 1: Path Expressions - ABAP Keyword Documentation

Explanation
The <source> clause in the CDS View Entity Data Definition can be used to specify the data source for the view entity. The <source> clause can accept different types of data sources, depending on the type of the view entity1.
A database table from the ABAP Dictionary: This is a valid type of data source for a CDS View Entity Data Definition. A database table from the ABAP Dictionary is a table that is defined in the ABAP Dictionary using the keyword TABLE or TABLE OF. The name of the database table must be unique within its namespace and must not contain any special characters2.
A CDS DDIC-based view: This is also a valid type of data source for a CDS View Entity Data Definition. A CDS DDIC-based view is a view that is defined in the Core Data Services using the keyword DEFINE VIEW ENTITY. The name of the CDS DDIC-based view must be unique within its namespace and must not contain any special characters3.
You cannot do any of the following:
An external view from the ABAP Dictionary: This is not a valid type of data source for a CDS View Entity Data Definition. An external view from the ABAP Dictionary is a view that is defined in an external application using any language supported by SAP, such as SQL, PL/SQL, or Java. The name of the external view must be unique within its namespace and must not contain any special characters4.
A database view from the ABAP Dictionary: This is not a valid type of data source for a CDS View Entity Data Definition. A database view from the ABAP Dictionary is a view that is defined in an external application using any language supported by SAP, such as SQL, PL/SQL, or Java. The name of the database view must be unique within its namespace and must not contain any special characters4.
References: 1: CDS DDL - DEFINE VIEW ENTITY - ABAP Keyword Documentation - SAP Online Help 2:
ABAP Dictionary Tables - SAP Online Help 3: CDS DDL - DEFINE VIEW ENTITY - ABAP Keyword Documentation - SAP Online Help 4: ABAP Dictionary Views - SAP Online Help

Explanation
The code snippet in the image is an example of using the FOR statement to create an internal table with a constructor expression. The FOR statement introduces an iteration expression that runs over the content of source_itab and assigns each row to the variable row. The variable row is then used to populate the fields of target_itab12. Some of the correct statements about the code snippet are:
FOR defines a loop that runs over the content of source_itab: This is true. The FOR statement iterates over the rows of source_itab and assigns each row to the variable row. The iteration expression can also specify a range or a condition for the loop12.
row is only visible within the loop: This is true. The variable row is a local variable that is only visible within the scope of the iteration expression. It cannot be accessed outside the loop12.
You cannot do any of the following:
source_itab is only visible within the loop: This is false. The variable source_itab is not a local variable that is defined by the FOR statement. It is an existing internal table that is used as the data source for the iteration expression. It can be accessed outside the loop12.
row is a predefined name and cannot be chosen arbitrarily: This is false. The variable row is not a predefined name that is reserved by the FOR statement. It is a user-defined name that can be chosen arbitrarily. However, it must not conflict with any existing names in the program12.
References: 1: FOR - Iteration Expressions - ABAP Keyword Documentation - SAP Online Help 2: ABAP 7.4 Syntax - FOR Loop iteration | SAP Community

Explanation
To redefine a component of a superclass in a subclass, you need to do the following12:
You add the clause REDEFINITION to the component declaration in the subclass. This indicates that the component is inherited from the superclass and needs to be reimplemented in the subclass. The redefinition must happen in the same visibility section as the component declaration in the superclass.
For example, if the superclass has a public method m1, the subclass must also declare the redefined method m1 as public with the REDEFINITION clause.
You implement the redefined component in the subclass. This means that you provide the new logic or behavior for the component that is specific to the subclass. The redefined component in the subclass will override the original component in the superclass when the subclass object is used. For example, if the superclass has a method m1 that returns 'Hello', the subclass can redefine the method m1 to return 'Hi' instead.
You cannot do any of the following:
You implement the redefined component for a second time in the superclass. This is not possible, because the superclass already has an implementation for the component that is inherited by the subclass. The subclass is responsible for providing the new implementation for the redefined component, not the superclass.
You add the clause REDEFINITION to the component in the superclass. This is not necessary, because the superclass does not need to indicate that the component can be redefined by the subclass. The subclass is the one that needs to indicate that the component is redefined by adding the REDEFINITION clause to the component declaration in the subclass.
References:1:METHODS - REDEFINITION - ABAP Keyword Documentation - SAP Online Help2:Redefining Methods - ABAP Keyword Documentation - SAP Online Help

Explanation
The response will be an activation error because the field names of the union do not match. This is because the field names of the union must match in order for the definition to be activated. The union operator combines the result sets of two or more queries into a single result set. The queries that are joined by the union operator must have the same number and type of fields, and the fields must have the same names1. In the given code, the field names of the union do not match, because the first query has the fields carrname, connid, cityfrom, and cityto, while the second query has the fields carrname, carrier_id, cityfrom, and cityto. The field connid in the first query does not match the field carrier_id in the second query. Therefore, the definition cannot be activated.
References: 1: UNION - ABAP Keyword Documentation