Airfoil efficiency at various angles of attack is expressed by the ratio of:

The correct answer is the ratio of lift to drag, which is a critical measure of airfoil efficiency. This ratio, often referred to as the lift-to-drag ratio (L/D), indicates how effectively an airfoil generates lift compared to the drag it produces. A higher lift-to-drag ratio signifies that the airfoil is more efficient, as it can produce more lift for a given amount of drag.

Understanding this concept is crucial in aerodynamics, as optimizing the lift-to-drag ratio allows for better performance in various flight conditions, enhancing aircraft maneuverability and fuel efficiency. When an airfoil operates at different angles of attack, the lift and drag forces change, making the evaluation of their ratio essential to assess performance.

Other options do not accurately reflect the efficiency of an airfoil. For instance, lift to dynamic pressure represents a different aerodynamic parameter related to coefficient calculations rather than efficiency itself, lift to weight addresses the aircraft's ability to operate under gravity rather than performance efficiency, and lift to velocity does not provide a clear measure of performance as it mixes different types of physical quantities.