Explanation
You would use Access Controls with CDS Views for the following reasons:
A). Only the data corresponding to the user's authorization is transferred from the database to the application layer. This is true because Access Controls allow you to define CDS roles that specify the authorization conditions for accessing a CDS view. The CDS roles are evaluated for every user at runtime and the system automatically adds the restrictions to the selection conditions of the CDS view.
This ensures that only the data that the user is authorized to see is read from the database and transferred to the application layer. This improves the security and the performance of the data access1.
C). You do not have to remember to implement AUTHORITY CHECK statements. This is true because Access Controls provide a declarative and centralized way of defining the authorization logic for a CDS view. You do not have to write any procedural code or use the AUTHORITY CHECK statement to check the user's authorization for each data source or field. The system handles the authorization check automatically and transparently for you2.
The following reasons are not valid for using Access Controls with CDS Views:
B). The system field sy-subrc is set, giving you the result of the authorization check. This is false because the system field sy-subrc is not used by Access Controls. The sy-subrc field is used by the AUTHORITY CHECK statement to indicate the result of the authorization check, but Access Controls do not use this statement. Instead, Access Controls use CDS roles to filter the data according to the user's authorization2.
D). All of the data from the data sources is loaded into your application automatically and filtered there according to the user's authorization. This is false because Access Controls do not load all the data from the data sources into the application layer. Access Controls filter the data at the database layer, where the data resides, and only transfer the data that the user is authorized to see to the application layer. This reduces the data transfer and the memory consumption of the application layer1.
References: 1: Access Controls | SAP Help Portal 2: ABAP CDS - Access Control - ABAP Keyword Documentation