Question: What triggers the network to start the timer T3460 during the authentication procedure?
Option 1: Receipt of an AUTHENTICATION RESPONSE message from the UE
Option 2: Sending an AUTHENTICATION REQUEST message to the UE and starting the authentication procedure
Option 3: Expiration of a previously set timer related to authentication
Option 4: Detection of a security breach in the NAS signalling connection
Explanation: According to the specified procedure, when the network initiates an authentication procedure, it sends an AUTHENTICATION REQUEST message to the UE and starts the timer T3460. This indicates that the correct trigger for starting the timer is indeed the sending of the AUTHENTICATION REQUEST message.