Which of the following protocols uses TCP instead of UDP and is incompatible with all previous versions?

A.
TACACS

B.
XTACACS

C.
RADIUS

D.
TACACS+

Explanation:
TACACS+ is not compatible with TACACS and XTACACS, and makes use of TCP.
Incorrect Answers:
A, B: TACACS and XTACACS make use of TCP and UDP.
C: RADIUS makes use of UDP.

http://en.wikipedia.org/wiki/TACACS