Final answer:
Airplanes may yaw during a power change in flight due to the torque effect and associated slipstream effect, which causes a yawing moment. As power settings are changed, the propeller generates a spiraling airflow, impacting the tail surface asymmetrically and resulting in yaw.
Step-by-step explanation:
Airplanes may yaw during a power change in flight primarily due to the torque effect and the associated slipstream effect. When power settings are adjusted, particularly in single-engine propeller aircraft, the engine and propeller produce a rotating 'wash' of air that spirals around the fuselage, striking the vertical tail surface asymmetrically. This interaction causes a yawing moment, turning the aircraft to the left or the right. The aileron controls rolling, not yaw, and the elevator affects pitch, not yaw.
Regarding the options provided in the question, b) Torque effect is the correct answer. Torque is the rotational equivalent of a force that tends to cause rotational acceleration. In aviation, torque can also cause an aircraft to roll or pitch in the opposite direction of the propeller's rotation due to Newton's Third Law, in addition to causing yaw due to asymmetric propeller-induced airflow.