Ailerons are described as:

• A. Control pitch by moving the elevator on the tail.
• B. Control yaw by the rudder.
• C. Primary flight control surfaces on the trailing edge of the airplane wing near the tip that control roll about the longitudinal axis.
• D. Increase drag via spoilers.

Answer: (C)

Ailerons control roll about the airplane’s longitudinal axis. They’re the primary flight control surfaces on the trailing edge of each wing near the tips and move in opposite directions: one wing’s aileron goes up while the other goes down. This differential movement changes lift between the wings—one wing loses lift while the other gains lift—causing the airplane to roll. Elevator and rudder control pitch and yaw, respectively, and spoilers are used mainly to add drag (and can assist with descent or roll in some designs) but are not the primary means of producing roll.