JAVA CERTIFICATION QUESTION: Functional interfaces and error handling in Java

Imagine that you are developing a high-load message board application  that will use nonblocking communication with a database.

This communication will be handled using the following service class:

Which functional interface could be used to expose the MsgBoardService getCommentCount method? Choose one.

A. Function

B. Supplier

C. IntUnaryOperator

D. Callable

E. A custom interface