Question: What triggers the multicast service deactivation procedure, and what is the purpose of this procedure?
Option 1: The UE sends an IGMP (IPv4) or MLD (IPv6) Leave message to leave a particular multicast service, and the purpose is to terminate the user registration to that MBMS multicast service.
Option 2: The BM-SC initiates the deactivation due to service-specific reasons, and the purpose is to update the MBMS bearer service list.
Option 3: The GGSN sends a STR to the BM-SC to deactivate a multicast service, and the purpose is to modify the APN configuration.
Option 4: The SGSN receives an MBMS UE Context Deactivation Request from the GGSN, and the purpose is to delete the MBMS UE Context.
Explanation: The correct answer is Option 1 because the multicast service deactivation procedure is triggered by the UE sending an IGMP (IPv4) or MLD (IPv6) Leave message to leave a particular multicast service. This procedure aims to terminate the user registration to that MBMS multicast service, as stated in the first sentence of the provided text.