Which condition must be met for an aircraft to generate lift?

For an aircraft to generate lift, the wings must achieve a critical angle of attack. This critical angle is the point at which the airflow over the wing begins to create enough pressure difference between the upper and lower surface of the wing to allow for lift.

As the angle of attack increases, the lift also increases up to this critical point. Beyond this angle, if the angle of attack continues to increase, the airflow can separate from the wing's upper surface, leading to a stall, which significantly reduces lift. Thus, maintaining the wings at or below this critical angle is crucial for effective lift production.

Other conditions like speed, thrust, and flight path direction play important roles in the overall performance and efficiency of the aircraft. However, the fundamental requirement for generating lift directly relates to the angle of attack of the wings, confirming that the correct answer revolves around the significance of the critical angle of attack in generating lift.