Which of the following are considered secondary flight controls?

Secondary flight controls are systems that supplement the primary flight controls of an aircraft, which include the ailerons, elevator, and rudder. These secondary systems enhance the aircraft's performance, efficiency, and handling characteristics.

Flaps and spoilers serve different purposes but are both classified as secondary flight controls. Flaps increase lift during takeoff and landing, allowing for slower speeds and shorter distances. Spoilers help to reduce lift and increase drag, providing quicker descent rates and enhancing roll control.

Aileron and rudder trims are adjustments made to the primary flight controls that allow the pilot to maintain a straight flight path without having to constantly apply pressure on the control yoke or stick. By trimming these controls, pilots can alleviate the physical demands on them during flight, making for a more comfortable experience.

The horizontal stabilizer control, often adjustable in certain aircraft, can aid in stabilizing pitch without requiring significant input from the elevator. This is particularly useful in varying weight and balance configurations.

The inclusion of all these components as secondary flight controls emphasizes the broad range of systems that work together to enhance an aircraft's handling and operational capabilities beyond mere directional control. Thus, the correct answer encompasses all options, confirming their role in the secondary flight control category.