Which of the following is NOT a type of flap?

Folding is not recognized as a traditional type of flap used in aircraft design. Flaps are devices that extend from the trailing edge of the wings and are designed to increase lift while decreasing stall speed during takeoff and landing. The other options are all established types of flaps.

Plain flaps are simple flaps that pivot down from the wing, increasing the wing's camber and thus generating more lift. Fowler flaps not only extend downward but also move backward, increasing the wing area and further enhancing lift. Slotted flaps incorporate a gap between the flap and the wing; this slot allows high-energy air to flow from the wing over the flap, helping to delay airflow separation and enhancing lift at higher angles of attack.

Folding, on the other hand, refers to a design feature that allows wings to fold—usually for storage purposes on aircraft carriers—not a specific type of flap that contributes to aerodynamic performance.