Question: What method is used for mutual authentication of the SCP and network functions if the PLMN uses protection at the transport layer?
Option 1: Mutual authentication provided by the transport layer protection solution
Option 2: Implicit authentication by NDS/IP or physical security
Option 3: Local authorization policy
Option 4: Co-location based implicit authentication
Explanation: According to the security architecture and procedures for 5G System, if the PLMN uses protection at the transport layer, authentication provided by the transport layer protection solution is used for mutual authentication of the SCP and network functions. This ensures secure communication between the SCP and network functions when transport layer protection is in place.