Which force acts horizontally against the thrust produced by an aircraft's engines?

The correct choice is the force that acts horizontally against the thrust produced by an aircraft's engines, which is drag. Drag is a resistance force that opposes the aircraft's forward motion through the air and is a critical factor in understanding an aircraft's performance.

When an aircraft is in flight, it generates thrust through its engines to move forward. However, as it moves, it encounters air which creates drag. Drag can be influenced by several factors, including the shape of the aircraft, its speed, and the air density. This force needs to be overcome for the aircraft to accelerate or maintain its speed effectively; otherwise, the engines must produce additional thrust to counteract the drag.

Other forces, such as lift, act vertically and are concerned with the aircraft's ability to rise or remain in the air, while weight is the force due to gravity acting downward. Relative wind refers to the airflow over the aircraft and is not a force itself but can enhance understanding of how lift and drag interact with the aircraft during flight. Thus, drag being the force acting horizontally against engine thrust is essential for analyzing aircraft performance dynamics.