Question: In an EFEPSNSC (EPS NAS Security Context) file, how is the validity of a stored ASME (Authentication and Key Agreement for Multimedia Services over IMS) key determined by the Mobile Equipment (ME)?
Option 1: By checking if the KSIASME (Key Set Identifier for ASME) matches the service number '85'.
Option 2: By verifying the length of the ASME key TLV is not set to '00', or the KSIASME indicates an available ASME key.
Option 3: By examining if the uplink and downlink NAS counts are synchronized.
Option 4: By checking if the identifiers of selected NAS integrity and encryption algorithms match the network's configuration.
Explanation: According to the specifications, the ME treats an ASME key as invalid if either the ASME key set identifier (KSIASME) indicates no ASME key is available or if the length indicated in the ASME key TLV is set to '00'. This ensures the ME only uses valid security contexts for EPS NAS security.