Answer:
The correct answer is letter "A": RADIUS.
Step-by-step explanation:
Introduced in 1991 the Remote Authentication Dial-In User Service or RADIUS, is an authentication and authorization network protocol helpful to grant a user access to a remote network. RADIUS serves are usually partnered with a core identity provider database. As a user tries to access remote access, RADIUS network requests the user to provide the credentials. RADIUS confirms the credentials in the database and if they match, permission is granted.