Which of the following is a key factor affecting an aircraft's fuel efficiency?

The total weight of the aircraft is a key factor affecting fuel efficiency because the heavier an aircraft is, the more lift is required to keep it airborne. This increase in lift demands more power from the engines, resulting in higher fuel consumption. Aircraft designers aim to manage weight carefully by considering various factors such as fuel, cargo, passengers, and structure, optimizing weight to achieve better fuel efficiency.

The other factors listed, while they may have an influence in specific contexts, are not as fundamental to fuel efficiency. The color of the aircraft generally has no significant impact on its aerodynamic performance or fuel consumption. The type of landing gear may affect drag slightly but does not play a critical role in overall fuel efficiency like weight does. Lastly, while the material of the wings can affect aerodynamics and overall performance, the immediate factor of weight plays a more direct role in determining how much energy is required for flight, making it the critical consideration in fuel efficiency.