Which of the following is a primary flight control?

Ailerons are classified as primary flight controls because they are directly responsible for controlling the aircraft's roll and lateral stability during flight. When the pilot moves the control yoke or stick to the left or right, the ailerons deflect in opposite directions—one aileron goes up while the other goes down. This differential movement creates an imbalance in lift along the wings, allowing the aircraft to bank and turn.

In contrast, flaps are secondary flight controls used primarily to increase lift during takeoff and landing, thereby allowing for lower speeds and shorter runway distances. Spoilers are also secondary devices that disrupt airflow over the wings to reduce lift and increase descent rate, usually during landing. Trim, while important for easing the pilot's control inputs by stabilizing the aircraft at a desired attitude, is not a primary flight control. Trim merely assists in maintaining control but does not directly maneuver the aircraft in the way that ailerons do. Thus, ailerons serve as fundamental controls that pilots use to navigate and execute turns effectively.