Which of the following is NOT one of the four forces acting on an airplane in flight?

In the context of aviation and the forces acting on an airplane in flight, lift, gravity, and drag are the fundamental forces that influence an aircraft's flight dynamics. Lift is the upward force generated by the wings as air flows over them, enabling the aircraft to rise. Gravity is the downward force that pulls the airplane towards the Earth due to its mass. Drag is the resistance force encountered by the aircraft as it moves through the air, acting opposite to the direction of flight.

Torque, while it can play a role in airplane dynamics, specifically in terms of engine rotation and the yawing motion caused by propeller-driven engines, is not classified among the primary forces acting on an airplane's overall flight. Instead, torque is a secondary effect resulting from other forces at play, particularly during takeoff and engine operation. Therefore, the option that does not belong to the primary forces acting on an airplane in flight is torque.