CompTIA Server+

Answer - C - Challenge Handshake Authentication Protocol (CHAP) sends a challenge to the originating client.

Key Takeaway: CHAP is an authentication scheme used by Point-to-Point Protocol servers to validate the identity of remote clients. CHAP periodically verifies the identity of the client by using a three-way handshake. CHAP sends a challenge to the originating client. This happens at the time of establishing the initial link and may happen again at any time afterwards. This challenge is sent back to the server and the encryption results are compared. If the challenge is successful, the client is logged on. Kerberos authentication uses a Key Distribution Center (KDC) to carry out the process. PAP offers no security at all.