Which of the following is considered a primary control surface on an aircraft?

The rudder is considered a primary control surface on an aircraft because it plays a crucial role in controlling the aircraft's yaw, which is the left and right movement of the nose. The rudder is located on the vertical tail surface and is operated by the pilot using foot pedals. By deflecting the rudder to one side, the pilot can cause the aircraft to turn in that direction, helping to maintain coordinated flight, especially during turns or in response to crosswinds.

Primary control surfaces, including the rudder, ailerons, and elevators, are essential for the basic maneuverability and stability of the aircraft. While the other options like a variable-pitch propeller and flaps serve important functions—such as enhancing performance for takeoffs and landings—their roles do not equate to the primary control functions provided by the rudder, ailerons, and elevators. This distinction emphasizes the fundamental nature of the rudder's contribution to an aircraft's control system.