Which of the following is a sample of secondary flight controls?

Secondary flight controls are devices that assist the primary flight controls in managing the aircraft's performance and stability but are not essential for basic control during flight. The correct answer includes spoilers, high lift devices, and the trimmable horizontal stabilizer (THS).

Spoilers are surfaces on the wings that can be deployed to disrupt airflow and reduce lift, which aids in descent and can also be used for roll control. High lift devices, such as flaps, enable the aircraft to fly safely at lower speeds by increasing the wing's lift during takeoff and landing phases. The trimmable horizontal stabilizer adjusts the aircraft's pitch attitude and helps maintain stable flight without continuous control inputs from the pilot.

The other choices listed do not comprise secondary flight controls. Winglets and fuselage stabilizers primarily serve to enhance aerodynamic efficiency and stability but do not have the same functional role as the devices mentioned in the correct response. Canards and control surfaces are more closely associated with primary flight controls, which directly affect the aircraft's attitude and direction. Lastly, while flaps are a type of lift device, rudder fins are not typically categorized under secondary controls, as rudder fins are more associated with directional control rather than stability enhancement.