What causes a stall in an aircraft?

A stall in an aircraft primarily occurs due to a combination of factors related to the angle of attack (AOA). When an aircraft's AOA is increased, initially, it can lead to an increase in lift; however, there comes a critical point where the airflow over the wing cannot remain attached, causing a rapid decrease in lift. This phenomenon is known as a stall.

The critical angle of attack is the threshold beyond which this loss of lift occurs. As the AOA exceeds this critical value, the lift decreases sharply and can lead to a stall. Thus, both the increase in AOA and the resulting reduction in lift when the critical angle is surpassed are critical in understanding stalls.

This reason underscores why the answer encompassing both the concepts of decreased lift resulting from an excessive angle of attack is deemed correct. It captures the comprehensive nature of stall mechanics, emphasizing the role of both an increase in AOA up to a point and the consequences of exceeding that point.