Final answer:
Light weight and high speed create the highest gust loading on an aircraft because a lighter aircraft is more susceptible to acceleration from wind gusts, and the loading effect increases with speed.
Step-by-step explanation:
The combination of aircraft weight and speed that causes the highest gust loading is light weight and high speed. Gust loading on an aircraft is affected by both the speed of the aircraft and its weight. When an aircraft encounters a gust of wind, the resulting load on the aircraft is a function of the dynamic pressure which is proportional to the square of the speed. If an aircraft is light, it will experience greater acceleration due to the gust, which in turn increases the loading effect. Hence, a lighter aircraft at high speed will experience the highest gust loading, as the force of the gust acting on the aircraft will cause a significant change in velocity, magnifying the load effect.