Question: What is the specific format requirement for the realm part of an Alternative Network Access Identifier (NAI) as per 3GPP specifications?
Option 1: The realm part should be a valid domain name.
Option 2: The realm part must match the operator's network identifier.
Option 3: The realm part shall be "unreachable.3gppnetwork.org".
Option 4: The realm part can be any string chosen by the user.
Explanation: According to 3GPP specifications, the Alternative NAI must contain a specific realm part to ensure it is not routable from any AAA server and to identify it as an alternative identifier. This requirement ensures that the Alternative NAI can be distinguished from other types of NAIs and handled appropriately in telecom networks.