Question: What triggers the GGSN to send an AAR message to the BM-SC during the MBMS Registration procedure?
Option 1: When the GGSN has an existing MBMS Bearer Context for an MBMS bearer service.
Option 2: When the GGSN receives an MBMS Registration from an SGSN for an MBMS bearer service, and it has no MBMS Bearer Context for that service.
Option 3: When the first MBMS UE Context is created in the GGSN for an MBMS bearer service for which the GGSN has an existing MBMS Bearer Context.
Option 4: When the BM-SC initiates the Session Start procedure with the GGSN.
Explanation: The correct answer is Option 2 because, according to the text, when the GGSN receives an MBMS Registration from an SGSN for an MBMS bearer service and it has no MBMS Bearer Context for that service, or when the first MBMS UE Context is created in the GGSN for an MBMS bearer service for which the GGSN has no MBMS Bearer Context, the GGSN sends an AAR message to the BM-SC. This indicates that the absence of an MBMS Bearer Context and the receipt of an MBMS Registration or creation of an MBMS UE Context are the triggers for sending the AAR message.