Which of the following forces acts on an aircraft in flight?

Lift is a crucial aerodynamic force that acts on an aircraft in flight, enabling it to ascend and remain airborne. It is generated primarily by the wings as they interact with the air. The specific design of the wing, known as an airfoil, facilitates this interaction by creating differences in air pressure above and below the wing, resulting in an upward force known as lift.

While gravity also acts on an aircraft by pulling it downward towards the Earth, it is not a force that is specifically associated with the task of keeping the aircraft in flight, making lift the more relevant choice in the context of flight dynamics. Magnetism does not have a significant influence on an aircraft's ability to fly, nor does friction. Instead, friction is a secondary force that comes into play primarily during takeoff, landing, and when the aircraft moves against the air but does not actively contribute to the forces that allow sustained flight.

Thus, the recognition of lift as the force that overcomes gravity during flight highlights its vital role in allowing aircraft to operate successfully in the skies.