Question: What is the primary reason for a UE to enter the EMM-DEREGISTERED.ATTEMPTING-TO-ATTACH substate?
Option 1: The UE has successfully attached to the network.
Option 2: The UE has received an explicit detach request from the network.
Option 3: The UE is in a state of limited service.
Option 4: The attach or combined attach procedure failed due to a missing response from the network.
Explanation: When the UE attempts to attach to the network but fails to receive a response, it enters the EMM-DEREGISTERED.ATTEMPTING-TO-ATTACH substate. This is because the UE needs to re-attempt attachment or take alternative actions due to the lack of response from the network.