What force opposes motion and holds an aircraft back?

The correct choice is related to the concept of drag, which is a fundamental aerodynamic force that acts opposite to the direction of an aircraft's motion. When an aircraft is in flight, it encounters resistance due to air molecules—this resistance is what we call drag. It acts parallel to the relative wind and works against the aircraft's thrust, thereby tending to slow it down.

Understanding drag is crucial for pilots and those involved in aviation as it influences how much thrust is necessary to maintain speed and control during various phases of flight. The design of aircraft often involves optimizing shapes to minimize drag in order to enhance fuel efficiency and overall performance.

The other forces mentioned play different roles: thrust propels the aircraft forward, lift enables it to rise against gravity, and weight pulls it down towards the earth. Therefore, drag is uniquely characterized as the opposing force that directly acts against the motion of the aircraft.