What physical phenomenon directly causes an aircraft wing to stall?

1. The angle of attack exceeds the critical angle of attack, causing the airflow to separate from the upper surface of the wing.
2. The aircraft's airspeed falls below the maximum cruising speed, causing engine flameout.
3. The pilot deploys full flaps at high speed, creating excessive parasite drag.
4. The center of gravity shifts too far forward, locking the elevators.