Which of the following is used by RADIUS for communication between clients and servers?

TCP (Transmission Control Protocol) is a connection-oriented protocol, which RADIUS does not natively use for its primary functions.

SSL (Secure Sockets Layer) is a cryptographic protocol for secure communication, often layered over transport protocols, but it's not the underlying transport protocol itself.

RADIUS (Remote Authentication Dial-In User Service) primarily uses User Datagram Protocol (UDP) for its communication. Specifically, it uses UDP port 1812 for authentication and UDP port 1813 for accounting messages, offering a connectionless and efficient transport for its request-response model.

SSH (Secure Shell) is a protocol for secure remote access and file transfer, unrelated to RADIUS's authentication and accounting functions.