When can N4567B be assigned 12,000 feet if N1234N is descending from 12,000 feet?

The correct choice states that N4567B can be assigned 12,000 feet when N1234N reports leaving 12,000 feet. This is because air traffic control regulations require a vertical separation of at least 1,000 feet between aircraft operating at the same altitude. When N1234N reports leaving 12,000 feet, it implies that it is transitioning to a lower altitude and has vacated the airspace at 12,000 feet.

At that moment, N4567B can be safely assigned the altitude of 12,000 feet, as it will not conflict with N1234N's flight path. Assigning 12,000 feet while N1234N is still at or near the altitude could lead to loss of separation and potential safety issues. Understanding air traffic communication protocols is crucial, as clear reporting from pilots helps controllers maintain safe distances between aircraft.

Other responses imply moments where N1234N is either still at or very close to 12,000 feet, which does not provide the necessary separation for N4567B to safely assume that altitude.