The secondary flight controls require ____ to operate.

The secondary flight controls on the A320, such as the slats, flaps, and spoilers, require hydraulic pressure to operate effectively. These controls are powered by the aircraft's hydraulic system, which provides the necessary force to move these devices. The use of hydraulic pressure allows for precise and responsive control over the aircraft's aerodynamic surfaces, enhancing its performance during various phases of flight, particularly during takeoff and landing.

Hydraulic systems are key in modern aircraft design, providing the power needed to operate larger and more complex control surfaces that would be impractical to operate manually or through other power sources. The functionality of secondary flight controls through hydraulic pressure is a significant aspect of flight safety and operational efficiency.

In contrast, the options related to electrical current, aerodynamic force, and pneumatic pressure do not align with the specific operation requirements of the secondary flight controls in the A320.