An employee is working from home and has to dial into the system. The employee is able to connect to the network but is unable to authenticate. Which of the following is MOST likely offline?

If the router were offline, the employee would be unable to reach the network at all, not merely fail at the authentication stage.

RADIUS (Remote Authentication Dial-In User Service) is the standard protocol used to centrally authenticate users who dial in or connect via VPN. The fact that the employee can connect to the network confirms that lower-layer infrastructure such as the router and DHCP server are functional. A failed authentication step with an active network connection points directly to the RADIUS server being unavailable to validate the user's credentials.

If the DHCP server were offline the employee would likely fail to obtain an IP address and would not be able to establish a network connection in the first place, which contradicts the scenario.

A firewall being offline would typically affect connectivity broadly or block specific traffic, but would not specifically cause an authentication failure while allowing a network connection to succeed.