What controls yaw about the vertical axis in an aircraft?

Yaw about the vertical axis in an aircraft is primarily controlled by the rudder. The rudder is a vertical stabilizer located on the tail of the aircraft and its primary function is to manage lateral movement or yaw. When the rudder is deflected left or right, it creates differential airflow over the tail surface, thus yawing the aircraft in the desired direction.

The ailerons, located on the wings, control roll about the longitudinal axis, which is different from yaw. Elevators are responsible for pitch control, allowing the aircraft to climb or descend along the lateral axis. Flaps are used to increase lift during takeoff and landing but do not have a direct role in controlling yaw. Thus, the rudder is the correct answer as it directly influences the aircraft's motion around the vertical axis.