Question: Under what conditions does an SGSN send a Session Update message to a specific RNC during an ongoing MBMS Multicast service session, given that it had provided a list of Routing Areas (RAs) in the initial MBMS Session Start Request?
Option 1: When any User Equipment (UE) moves into a new RA within the existing list.
Option 2: When the SGSN's overall capacity for handling MBMS sessions changes.
Option 3: When either the first UE with an activated service enters a non-listed RA or the last UE with an activated service leaves a listed RA.
Option 4: Periodically, as a keep-alive mechanism to maintain session awareness.
Explanation: This option correctly identifies the specific conditions under which an SGSN initiates a Session Update procedure for MBMS Multicast services. These conditions are directly tied to changes in UE location relative to the listed RAs, ensuring efficient resource allocation and service continuity. The other options do not accurately reflect the triggers for a Session Update as per the described telecom scenario.