Which aerodynamic force opposes the motion of the aircraft?

The aerodynamic force that opposes the motion of the aircraft is drag. Drag is a resistance force that acts opposite to the direction of the aircraft's motion through the air, making it a critical factor in aerodynamics. It arises primarily from the interaction between the aircraft's surface and the air, including both skin friction and pressure differences that occur as the air flows around the aircraft.

In contrast, thrust is the forward force generated by the aircraft’s engines, which propels it into the air, while lift is the upward force that allows the aircraft to rise off the ground. Weight acts downward due to gravity, balancing the upward lift in flight but does not oppose the forward motion in the same way that drag does. Therefore, drag is the correct answer as it directly counteracts the thrust and slows down the aircraft.