What significantly increases an aircraft's stall speed?

An aircraft's stall speed is primarily influenced by its weight. When the weight of the aircraft increases, the stall speed must also increase to maintain level flight. This is due to the relationship between an aircraft's weight and the lift required to keep it airborne. The stall speed is defined as the minimum speed at which the aircraft can maintain level flight without stalling, which occurs when the wings fail to produce enough lift for the given weight.

As the weight of the aircraft increases, the wing needs to generate more lift to counteract the increased gravitational force. To achieve this higher lift, the aircraft must fly at a higher speed. The stall speed increases with weight according to the square root of the weight-to-lift equation, making it a crucial consideration for pilots during various phases of flight, especially during takeoffs and landings.

Other options do not contribute to an increase in stall speed: decreasing weight would lower stall speed, while changes in air density and humidity have different implications primarily related to lift and performance characteristics rather than directly affecting stall speed in the context of weight considerations.