Question: In the IP Multimedia Subsystem (IMS), when updating charging information, what triggers the Home Subscriber Server (HSS) to immediately push the new information to the Serving-Call Session Control Function (S-CSCF)?
Option 1: Change in User Equipment (UE) location
Option 2: Update of charging information for a registered Public Identity
Option 3: Expiration of a subscriber's service package
Option 4: Receipt of a new billing cycle notification
Explanation: This is correct because, according to the IMS protocol, if a Public Identity corresponding to the updated Private User Identity is registered (or was previously registered with an active S-CSCF storing the user profile), the HSS should promptly update the S-CSCF with the new charging information. This ensures seamless and accurate billing and service management for the subscriber's active sessions.