How does Vy change with regards to altitude?

The correct answer is that Vy decreases with an increase in altitude due to less air density affecting various aerodynamic factors. As altitude increases, the air becomes less dense, which directly impacts the aircraft's performance. Specifically, with lower air density, the lift produced by the wings decreases, and while drag also changes, Vy is primarily determined by the power available from the engines relative to the power required to maintain a certain flight path.

At higher altitudes, an aircraft requires a different climb speed, which can lead to a decrease in Vy as we ascend. This happens because the engine's power output can become less effective in producing the required thrust to overcome the increased drag and maintain the same rate of climb.

Other factors like lift requirements and drag do influence aircraft performance at varying altitudes, but the primary takeaway is that Vy's definition as the best rate of climb speed is sensitive to changes in air density and, consequently, decreases as altitude increases.