Question: What action is triggered at the Serving-Call Session Control Function (S-CSCF) upon receiving a re-registration request from a User Equipment (UE) in the context of network-initiated authentication for an already registered user?
Option 1: Update of UE's Location Information
Option 2: Initiation of a new IMS AKA (IP Multimedia Subsystem Authentication and Key Agreement) procedure to re-authenticate the user
Option 3: Immediate De-registration of the Subscriber
Option 4: Sending an SMS Notification for Security Alert
Explanation: In the context of network-initiated authentication, when a UE is already registered and the S-CSCF needs to re-authenticate the user, sending a request for re-registration triggers this. The S-CSCF then initiates a new IMS AKA procedure for re-authenticating the user, ensuring security by verifying credentials and establishing fresh keys if necessary.