Question: What method is used for authentication between network functions within one Public Land Mobile Network (PLMN) if the PLMN does not use protection at the transport layer?
Option 1: Token-based authorization
Option 2: Implicit authentication by NDS/IP or physical security
Option 3: Authentication provided by the application layer
Option 4: Certificate-based authentication
Explanation: According to the security architecture and procedures for 5G System, if a PLMN does not use protection at the transport layer, authentication between network functions within one PLMN may be implicit by Network Domain Security/Internet Protocol (NDS/IP) or physical security. This method is used as an alternative when transport layer protection is not employed.