Question: What triggers the start of the registration procedure for mobility and periodic registration update of 5GS services in a UE?
Option 1: Expiration of timer T3510
Option 2: Receipt of a paging request from the network
Option 3: Expiration of timer T3512
Option 4: Initiation of an emergency call
Explanation: The registration procedure for mobility and periodic registration update of 5GS services is started when timer T3512 expires, as stated in the text. This timer controls the periodic registration update of 5GS services in the UE.