N74Y has been cleared to descend from 8,000 feet to 4,000 feet. When can a higher aircraft be assigned 8,000 feet?

The correct answer is based on the principle of ensuring safe separation between aircraft at different altitudes during a descent. When N74Y is cleared to descend from 8,000 feet to 4,000 feet, it is important to maintain a safe vertical separation from any aircraft that might be assigned to the same altitude of 8,000 feet.

Assigning a higher aircraft to 8,000 feet is safe only when N74Y is at a point in its descent where it is clear of any potential conflict. Reporting leaving 7,000 feet indicates that N74Y is actively descending and has passed through an altitude that could potentially conflict with an aircraft at 8,000 feet. At this point, the vertical separation has been established, as N74Y is moving away from 8,000 feet, allowing for another aircraft to be assigned that altitude safely.

The other options suggest moments in the descent where assigning 8,000 feet might not ensure appropriate separation. For instance, waiting until N74Y reaches 6,000 feet, 5,000 feet, or nearing 4,000 feet, does not provide the same immediate assurance of vertical space between the descending aircraft and a new one at 8,000 feet.