Question: In which substate of EMM-DEREGISTERED is valid subscriber data not available for the UE?
Option 1: EMM-DEREGISTERED.NORMAL-SERVICE
Option 2: EMM-DEREGISTERED.NO-IMSI
Option 3: EMM-DEREGISTERED.ATTEMPTING-TO-ATTACH
Option 4: EMM-DEREGISTERED.ATTACHED-NO-IPV4-ADDR
Explanation: The substate EMM-DEREGISTERED.NO-IMSI is the only substate where valid subscriber data are not available for the UE, as indicated in the specification. This highlights the importance of IMSI (International Mobile Subscriber Identity) in identifying and authenticating subscribers in the EPS network.