What action does a RADIUS server take when it cannot authenticate the credentials of a user?

What action does a RADIUS server take when it cannot authenticate the credentials of a user?

A.
An Access-Reject message is sent.

B.
An Access-Challenge message is sent, and the user is prompted to re-enter credentials.

C.
A Reject message is sent.

D.
A RADIUS start-stop message is sent via the accounting service to disconnect the session.

Explanation: