Which statement is correct about ABAP methods?

The statement that METHOD... ENDMETHOD is mandatory is correct because, in ABAP, methods must be explicitly defined with these keywords to denote the beginning and the end of the method's implementation. This requirement ensures that the structure of the method is clear and the compiler can appropriately parse the method code. Every method in an ABAP class must have a defined body using these keywords, which gives a clear demarcation of the method's scope and functionality within the class.

Regarding the other statements, methods in ABAP can technically be defined in multiple locations, not just within the interface, and they don't necessarily have to return a value, as you can create void methods. While it is possible to have both private and public sections, this concept doesn't directly apply to the interface section of a class, making options A, C, and D inaccurate in this context.