CoA-ACK is the CoA response code that is sent if an authorization state is changed successfully on a Cisco IOS device. CoA-ACK stands for CoA acknowledgment, which indicates that the device has received and processed the CoA request from the server and applied the new authorization settings to the session. The attributes returned within a CoA-ACK can vary based on the CoA request, such as session reauthentication, session termination, or session modification. The other options are not correct because they are not valid CoA response codes. CoA-NCL, CoA-NAK, and CoA-MAV are not defined in RFC 5176, which specifies the CoA protocol. CoA-NAK is the closest option, but it stands for CoA non-acknowledgment, which indicates that the device has rejected the CoA request from the server due to some error or inconsistency. References := Some possible references are:
* RADIUS Change of Authorization - Cisco
* Security and VPN Configuration Guide, Cisco IOS XE 17.x
* RFC 5176 - Dynamic Authorization Extensions to Remote Authentication Dial In User Service (RADIUS)