Question: What action does the UE take when receiving a message containing an IE unknown in the message, but encoded as "comprehension required"?
Option 1: The UE ignores the message and continues with normal operation.
Option 2: The UE returns a status message (EMM STATUS or ESM STATUS depending on the PD) with cause #96 "invalid mandatory information".
Option 3: The UE tries to treat the message based on implementation-dependent actions.
Option 4: The UE sends an error message to the network and terminates the connection.
Explanation: According to the NAS protocol, when a UE receives a message containing an IE unknown in the message but encoded as "comprehension required", it diagnoses an error. In this case, if the message is not one of the specific messages listed in clause 7.5.3 (items a-d), the UE returns a status message with cause #96 "invalid mandatory information". This ensures that the network is informed about the error and can take corrective action.