Which of the following is a primary flight control surface?

The ailerons are indeed classified as primary flight control surfaces because they are crucial for controlling the roll of the aircraft around its longitudinal axis. By moving in opposite directions on the wings, the ailerons create a difference in lift between the two wings, allowing the aircraft to bank and turn. This rolling motion is essential for maneuvers during flight, such as turns.

In contrast, the fuselage is the main body of the aircraft and houses the crew, passengers, and cargo but does not directly influence flight control. The horizontal stabilizer is a fixed surface that provides stability but is not a primary control surface; it mainly contributes to pitch control along with the elevator, which is typically a secondary control surface. Flaps also serve an important role in flight, particularly during takeoff and landing, by increasing lift and drag, but they are not classified as primary flight control surfaces since they do not provide continuous control input throughout the flight as ailerons do.