Which of the following is NOT a primary flight control surface?

In the context of flight control surfaces, the primary control surfaces are critical for manipulating the aircraft's attitude and direction during flight. The ailerons, elevators, and rudders are all fundamental components that directly control roll, pitch, and yaw, respectively.

Ailerons are located on the wings and control the roll of the aircraft, allowing it to tilt left or right. The elevator, which is located on the tail, controls the pitch, enabling the aircraft to ascend or descend. The rudder, also situated on the tail, manages yaw and helps to stabilize the aircraft during flight.

Flaps, on the other hand, while essential for enhancing lift and improving aerodynamic performance during takeoff and landing, are considered secondary flight control surfaces. They are primarily used to increase the lift coefficient at slower speeds or to reduce the stall speed, rather than for actively controlling the aircraft's attitude in flight.

Choosing flaps as the answer indicates an understanding that while they contribute significantly to the aircraft's overall performance, they do not serve as primary flight controls like the other options listed.