Which of the following forces acts against thrust in an aircraft?

The force that acts against thrust in an aircraft is drag. Drag is the aerodynamic resistance that opposes an aircraft's motion through the air. It is caused by factors such as the shape of the aircraft, surface roughness, and the speed of the aircraft. As the aircraft moves forward, air resistance increases, creating drag that must be overcome by the thrust produced by the aircraft's engines.

Understanding drag is crucial for pilots and engineers alike, as it directly affects fuel efficiency, speed, and overall performance. In designing aircraft, minimizing drag is a key consideration to ensure that the engines do not have to work harder than necessary to maintain flight, thus improving operational efficiency.