At which speed is maximum excess thrust achieved?

The maximum excess thrust is achieved at the speed known as Vx, or the best angle of climb speed. This is the speed at which an aircraft can gain the most altitude in the shortest horizontal distance. At Vx, the difference between the thrust produced by the aircraft's engines and the drag acting on the aircraft reaches its peak, resulting in maximum excess thrust.

In the context of aerodynamics and performance, excess thrust is the amount of thrust available above what is required to maintain level flight. At Vx, the aircraft is climbing at a steep angle, where the power is utilized effectively to gain altitude, thus maximizing the excess thrust available for climb.

The other choices reflect different characteristics of aircraft performance but do not provide a peak in excess thrust. Vy is the best rate of climb speed, which focuses more on time to gain altitude rather than the distance. The best lift/drag ratio relates to the most efficient glide, and minimum power speed describes the speed at which minimum power is required to maintain level flight, not necessarily focusing on maximizing excess thrust.