Which of the following authentication services should be replaced with a more secure alternative?

RADIUS is a current and widely used AAA protocol that encrypts passwords using MD5 and is supported across most modern network infrastructure, though it has known limitations it is still considered acceptable.

The original TACACS protocol uses UDP and encrypts only the password field in authentication packets, leaving other session data exposed. It lacks the full packet encryption, TCP reliability, and granular command authorization that TACACS+ provides. TACACS (original) is considered obsolete and should be replaced with TACACS+ or RADIUS for modern environments.

TACACS+ is the modern, more secure successor that encrypts the entire authentication packet body (not just the password) and uses TCP, making it more reliable and secure.

XTACACS is an extended version of TACACS developed by Cisco that improved upon the original, though it has since been superseded by TACACS+.