What defines an airfoil?

An airfoil is specifically defined as a shape designed to produce lift when air flows over it. This is primarily due to the difference in air pressure created on either side of the airfoil as a result of the air moving over and underneath it. The curvature and angle of the airfoil are designed in such a way that, as the aircraft moves through the air, lift is generated, allowing the aircraft to ascend, maintain altitude, or descend safely.

The other choices, while related to the broader context of aircraft performance and design, do not accurately capture the primary function of an airfoil. A structural component of an aircraft exists for various purposes, including support and stability, but does not solely define what an airfoil is. A method of controlling pitch relates to flight control surfaces, such as the elevator, rather than lift generation by airfoils. Lastly, aerodynamic efficiency is a broader concept that encompasses various factors affecting the performance of an aircraft, including but not limited to the design of airfoils. Therefore, the defining characteristic of an airfoil is indeed its ability to provide lift from moving air.