Which surface of an airplane controls directional movement about the vertical axis?

The rudder is the primary control surface responsible for managing directional movement about the vertical axis of an airplane, which is also known as yaw. When the pilot applies right or left rudder, the rudder deflects in the corresponding direction, causing the nose of the airplane to move right or left. This is critical for maintaining coordinated flight, especially during turns, and helps to counteract adverse yaw created by the ailerons.

The rudder operates in conjunction with other control surfaces like ailerons and elevators to achieve effective overall control of the aircraft. Ailerons, while they do influence roll about the longitudinal axis, are not involved in controlling yaw. Elevators are primarily responsible for pitch control about the lateral axis. The stabilator, which is a type of all-moving elevator, also controls pitch rather than yaw. Therefore, the rudder is the correct answer when it comes to controlling directional movements about the vertical axis.