What primarily causes a stall in an aircraft?

A stall in an aircraft is primarily caused by exceeding the critical angle of attack, which leads to a loss of lift. The critical angle of attack is the angle at which the airflow can no longer smoothly adhere to the wing surface, resulting in a turbulent airflow and a significant drop in lift production. When an aircraft exceeds this angle during any phase of flight, including straight and level flight, turns, or during climb or descent, it can no longer maintain sufficient lift to support its weight, leading to a stall.

Loss of power could contribute to a stall if it results in an inability to maintain airspeed, but it is not the primary cause of a stall itself; rather, it can exacerbate the situation. Rapid descent from altitude might increase the risk of a stall, but it is not the direct cause. Improper weight distribution can affect the aircraft's center of gravity and overall handling, but it does not directly lead to exceeding the critical angle of attack under normal circumstances.