Question: Under what condition does a User Equipment (UE) initiate an initial registration procedure in a 5G System?
Option 1: When the UE is in idle mode and receives a paging message.
Option 2: As soon as access is allowed in the selected cell for the UE, if still needed.
Option 3: When the UE performs a handover to a new cell.
Option 4: When the UE is powered on and has not registered with the network before.
Explanation: According to the NAS protocol for 5G System, the UE initiates the initial registration procedure as soon as access is allowed in the selected cell for the UE, if still needed. This ensures that the UE can establish a connection with the network and receive necessary configuration parameters.