N12B is at 6,000 feet and has been cleared for approach. When can another aircraft be assigned 6,000 feet?

The correct choice indicates that another aircraft can be assigned 6,000 feet when N12B reports leaving 5,000 feet. This situation is grounded in maintaining safe vertical separation between aircraft in controlled airspace.

When N12B is at 6,000 feet and is cleared for approach, it means the aircraft is beginning a descent. The critical point at which separation can be assured is when N12B reports leaving 5,000 feet; at this moment, it is confirmed that N12B is no longer occupying 6,000 feet, and the altitude becomes available for another aircraft.

Assigning 6,000 feet before N12B reaches 5,000 feet could lead to a violation of safe vertical spacing, compromising safety. Therefore, a report from N12B at 5,000 feet acts as an indicator for air traffic control that it's safe to clear another aircraft to the same altitude. This procedure is fundamental to ensure effective traffic management and safety in the airspace.