Question: In a 5G System, when the Network Exposure Function (NEF) notifies an Application Function (AF) of timer expiry for previously allocated MBS (Multicast/Broadcast Service) TMGI(s), what is the expected response from the AF upon successful reception?
Option 1: The AF sends a `Nnef_MBSTMGI_ExpiryNotify` request back to the NEF with an HTTP "200 OK" status code.
Option 2: The AF initiates a new subscription for MBS TMGI(s) using the HTTP PUT method.
Option 3: The AF responds with an HTTP "404 Not Found" status code indicating no knowledge of the TMGI(s).
Option 4: The AF sends a `Nnef_MBSTMGI_ExpiryNotify` response message with an HTTP "204 No Content" status code.
Explanation: Upon successful reception of the timer expiry notification for MBS TMGI(s), the Application Function (AF) is required to acknowledge this without providing additional content, hence responding with an HTTP "204 No Content" status code. This response indicates that the AF has successfully received and processed the notification without needing to return any content in the response body.