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

The correct answer is flaps because they are not classified as primary flight controls. Primary flight controls are essential for controlling the basic flight attitude and maneuvering of the aircraft. They include the ailerons, which control roll, the elevator, which controls pitch, and the rudder, which controls yaw.

Flaps, on the other hand, are secondary flight controls that are primarily used to increase lift at lower speeds, especially during takeoff and landing. Their main purpose is to enhance the aircraft's performance during these critical phases, rather than to directly control the aircraft's attitude in flight. Thus, flaps do not fit into the category of primary flight controls, making that the correct answer to the question.