Question: What triggers the EMM entity in the UE to initiate the UPLINK NAS TRANSPORT message procedure?
Option 1: Receipt of a downlink NAS transport message from the network.
Option 2: Request from the SMS entity to send an SMS message.
Option 3: Establishment of a new PDN connection.
Option 4: Receipt of a paging message from the network.
Explanation: The EMM entity in the UE initiates the UPLINK NAS TRANSPORT message procedure upon request from the SMS entity to send an SMS message, as specified in the NAS protocol for EPS. This highlights the role of the SMS entity in triggering the transport of NAS messages in the UE.