Wing flaps can alter lift most by: altering the air density and airspeed changing the cross sectional depth changing the approach angle altering the aerofoil shape of the wing

Wing flaps alter lift most significantly by altering the aerofoil shape of the wing. Here's why:

1. Altering the air density and airspeed: While these factors do affect lift, they are not directly controlled by the wing flaps. Air density and airspeed are more dependent on the aircraft's altitude, speed, and the weather conditions.

2. Changing the cross-sectional depth: The cross-sectional depth of the wing can affect lift, but it's not the primary function of the wing flaps. The depth of the wing is more or less a fixed characteristic of the aircraft's design.

3. Changing the approach angle: The approach angle, or angle of attack, does significantly affect lift. However, this is typically controlled by the aircraft's elevators (on the tail), not the wing flaps.

4. Altering the aerofoil shape of the wing: This is the primary function of the wing flaps. By extending the flaps, the pilot can change the wing's shape to create more lift, which is especially useful during takeoff and landing. The flaps increase the curvature of the wing, creating a higher pressure difference between the top and bottom of the wing, which increases lift.