Question: What is the rule regarding transmission of a 5G authentication vector to an EPS core network entity?
Option 1: It is permitted if the SUPI has the form of an IMSI.
Option 2: It is permitted only in mobility scenarios.
Option 3: It is not permitted, but unused EPS authentication vectors can be transmitted to a 5G core network entity.
Option 4: It is not permitted under any circumstances.
Explanation: According to the text, "The transmission of a 5G authentication vector to an EPS core network entity is not permitted." This rule ensures strict security separation between EPS and 5G domains.