What is the relationship between altitude and stall speed?

Stall speed is the minimum speed at which an aircraft can maintain level flight. As altitude increases, the air density decreases. This reduction in air density affects the aerodynamic forces acting on an aircraft, specifically the lift generated by the wings. Since stall speed is influenced by the need to generate sufficient lift to counteract the weight of the aircraft, a decrease in air density means that the wings must achieve a higher true airspeed to maintain that lift.

In summary, as altitude increases, stall speed increases because the aircraft requires a higher true airspeed to achieve the same amount of lift due to the thinner air. Thus, the correct answer accurately captures the relationship between altitude and stall speed.