Which of the following increases wing surface area?

The correct answer regarding which of these options increases wing surface area is Fowler flaps. Fowler flaps are designed to not only increase the camber of the wing, enhancing lift, but they also effectively extend the wing's surface area. This extension occurs because Fowler flaps slide rearward and downward when deployed, which increases the overall wing area and modifies the airflow over the wing, improving lift performance during takeoff and landing.

In contrast, leading edge slats mainly modify airflow over the wing's leading edge, improving stall characteristics without directly increasing the overall wing area. Trailing edge flaps also enhance lift by increasing the camber of the wing and can change the aerodynamic characteristics, but they do not extend the wing's surface area as significantly as Fowler flaps. Speed brakes are primarily used to reduce aerodynamic lift during descent or landing, which does not involve increasing wing surface area. Therefore, when considering the function of increasing the effective wing area, Fowler flaps are the most appropriate choice.