Which authentication service is needed to configure 802.1x?

Which authentication service is needed to configure 802.1x? 

A. RADIUS with EAP Extension 
B. TACACS+ 
C. RADIUS with CoA 
D. RADIUS using VSA 

Correct Answer: A 
Explanation 
Explanation/Reference: 
With 802.1x, the authentication server--performs the actual authentication of the client. The authentication server validates the identity of the client and notifies the switch whether or not the client is authorized to access the LAN and switch services. Because the switch acts as the proxy, the authentication service is transparent to the client. The Remote Authentication Dial-In User Service (RADIUS) security system with Extensible Authentication Protocol (EAP) extensions is the only supported authentication server.