What typically causes a stall in an aircraft?

A stall in an aircraft is primarily caused by the disruption of airflow over the wing. This disruption occurs when the angle of attack—the angle between the wing's chord line and the relative wind—exceeds a critical threshold. Beyond this angle, the smooth airflow over the wing begins to separate, resulting in a loss of lift.

When the airflow separates, the wing can no longer generate the lift necessary to keep the aircraft in flight, which leads to the stall condition. Pilots must be aware of this critical angle of attack during all phases of flight, especially during maneuvers that may increase the angle of attack, such as climbing steeply or making sharp turns.

The other options, while they may affect flight performance in some way, do not directly cause a stall in the classic sense. Increased engine power may improve climb performance but does not directly relate to the airflow over the wings. Low fuel levels and excessive weight in the cabin can affect overall aircraft performance and handling but are not specific factors causing a stall. The primary factor remains the disruption of the airflow over the wing, making that the correct and most relevant answer.