Final answer:
The best authentication protocol for untrusted networks is TACACS+ because it encrypts the entire authentication process and separates authentication, authorization, and accounting services, offering enhanced security over the other options.
Step-by-step explanation:
When selecting an authentication protocol for use over untrusted networks, the key is to ensure that the protocol provides strong authentication mechanisms and encryption. Here are the options you've provided:
Therefore, the best option for Joe would be TACACS+ because it provides better security features for authentication and encrypts the entire authentication process making it well suited for untrusted networks.