Question: What happens when a 5G authentication challenge sent by the network is rejected by the UE?
Option 1: The UE proceeds with a new 5G authentication challenge.
Option 2: The 5G AKA based primary authentication and key agreement procedure is terminated, and no further action is taken.
Option 3: The network initiates a security mode control procedure to take the existing partial native 5G NAS security context into use.
Option 4: The UE deletes all valid KAUSF and KSEAF keys.
Explanation: According to the text, However, the UE can reject the 5G authentication challenge sent by the network. This implies that if the UE rejects the challenge, the procedure is terminated, and no further action is taken.