What enables the stall warning horn to sound?

The correct answer pertains to the forward movement of the center of pressure, which directly influences the aerodynamic forces acting on the aircraft. The stall warning horn is triggered when the angle of attack increases to near the point of stall. This increase in angle of attack results in a shift of the center of pressure forward on the wing, which changes the airflow characteristics around the wing.

When the center of pressure moves forward, it tends to create a condition where the wing is approaching critical angles, prompting the stall warning system to activate. This design is crucial for enhancing pilot awareness that a stall is imminent, allowing for timely corrective actions to be taken.

The other choices do not directly relate to the mechanism that activates the stall warning system. Changes in altitude, for example, do not have a direct connection to the stall warning. The presence of ice can affect stall characteristics but is not the cause of the stall warning horn sounding. Likewise, while the position of control surfaces influences flight dynamics, it is the center of pressure moving forward due to angle of attack that specifically triggers the stall warning system.