Which of the following is an authentication service that uses UDP as a transport medium?

TACACS+ (Terminal Access Controller Access-Control System Plus) uses TCP port 49, providing connection-oriented, reliable transport - not UDP.

LDAP (Lightweight Directory Access Protocol) uses TCP port 389 (or TCP 636 for LDAPS) and is a directory service protocol, not primarily an authentication transport service.

Kerberos primarily uses TCP for larger tickets and UDP for smaller exchanges, but it is not described as a UDP authentication service and is not the answer being sought here.

RADIUS (Remote Authentication Dial-In User Service) uses UDP port 1812 for authentication and authorization, and UDP port 1813 for accounting. Its use of UDP makes it connectionless, which improves performance in high-volume environments at the cost of built-in reliability. This is a key distinguishing characteristic from TACACS+, which uses TCP.