Question: What methods can be used for authentication between Network Function (NF) and NRF in direct communication, if the Public Land Mobile Network (PLMN) does not use protection at the transport layer?
Option 1: Only mutual authentication provided by the application layer.
Option 2: Either mutual authentication provided by the transport layer or implicit authentication by NDS/IP.
Option 3: Mutual authentication provided by the session layer or physical security.
Option 4: Implicit authentication by Network Domain Security/Internet Protocol (NDS/IP) or physical security.
Explanation: According to the 3GPP TS 33.501 specification, if the PLMN does not use protection at the transport layer, mutual authentication of NRF and NF may be implicit by NDS/IP or physical security. This is a valid method for authentication in direct communication between NF and NRF.