Question: In a UE supporting both 3GPP and non-3GPP access in 5GS, what does an `<access_domain pref>` value of `0` indicate for MO SMS over NAS messages?
Option 1: Non-3GPP access in 5GS is always used for MO SMS.
Option 2: 3GPP access is exclusively preferred for all SMS services.
Option 3: The UE will randomly select between 3GPP and non-3GPP access for each MO SMS.
Option 4: 3GPP access is preferred, but non-3GPP access in 5GS is used if no 3GPP access is available.
Explanation: This option correctly interprets the `<access_domain pref>` value of `0` as indicating a preference hierarchy where 3GPP access is the first choice for MO SMS over NAS messages, with non-3GPP access in 5GS serving as a fallback when 3GPP access is unavailable, ensuring continuity of service while prioritizing 3GPP access when possible.