Which of the following is an authentication protocol that can be used for users in the field?

WEP (Wired Equivalent Privacy) is a deprecated wireless encryption protocol that protects data in transit over Wi-Fi; it is not an authentication protocol for verifying user identity.

WPA2 is a Wi-Fi security standard that provides data encryption and integrity for wireless networks; it is not a user authentication protocol for remote or field access.

RADIUS (Remote Authentication Dial-In User Service) is a client-server AAA protocol that provides centralized authentication for users connecting remotely - including field personnel using VPNs, dial-up, or wireless access points. It was purpose-built for authenticating users who are not on-premises, making it the correct protocol for the described scenario.

TKIP (Temporal Key Integrity Protocol) is an encryption protocol used within WPA to improve on WEP's key management weaknesses and does not perform user authentication.