What is the primary function of flaps on an aircraft?

The primary function of flaps on an aircraft is to extend and retract during different flight phases. Flaps are movable surfaces attached to the wings that increase the wing's surface area and change its shape, enhancing lift during critical phases of flight such as takeoff and landing. When flaps are deployed, they allow the aircraft to fly at lower speeds without stalling, which is crucial for safe operations in these phases. By improving lift at slower speeds, flaps help ensure that the plane can safely take off and land in shorter distances, making them a vital component of aircraft design and operation.

The other options relate to functions that are not directly associated with flaps. While flaps can contribute to better fuel efficiency indirectly due to improved aerodynamics at low speeds, their main role is not fuel efficiency during flight. Flaps do not adjust the altitude of the aircraft in a direct sense; rather, they assist in maintaining proper lift at lower velocities. Finally, flaps do not increase the speed of the aircraft; rather, they are used primarily to enhance lift and control at lower speeds.