Question: What action can a trusted application server indicate to the HPLMN in response to a notification when a UE successfully registers in a VPLMN?
Option 1: Reject UE registration without a specific cause
Option 2: Trigger a Steering of Roaming (SoR) command during the registration procedure
Option 3: Send an SMS to the UE with equipment and subscription identifiers
Option 4: Update the UE's subscription information in the HPLMN database
Explanation: The trusted application server can indicate specific actions to the HPLMN, such as sending an SMS to the UE, in response to a notification when a UE successfully registers in a VPLMN. This allows the HPLMN to provide personalized services or notifications to the UE based on its registration status and equipment/subscription identifiers.