What protocol was UDP based and mainly intended to provide validation of dial up user login passwords?

What protocol was UDP based and mainly intended to provide validation of dial up user login
passwords?

A.
PPTP

B.
L2TP

C.
IPSec

D.
TACACS

Explanation:
The original TACACS protocol was developed by BBN for MILNET. It was UDP based and mainly
intended to provide validation of dial up user login passwords. The TACACS
protocol was formally specified, but the spec is not generally available.