What causes your vehicle's speed to naturally increase on downgrades?

Answer: C — Gravity

When driving on downgrades, the natural increase in a vehicle's speed is primarily due to gravity. As the vehicle descends the slope, gravitational force pulls it downward, which accelerates the vehicle. This effect is particularly noticeable on steep declines, where the gravitational pull is stronger because of the slant of the road. In this scenario, the other options do not play a significant role in causing the vehicle's speed to increase. Increased throttle involves actively pressing the accelerator, which is not necessary for speed increase on a downgrade. Wind can affect the vehicle's stability and drag, but it does not inherently increase speed. Road conditions might impact braking distance or control but do not contribute to speed increase from gravity. Thus, gravity is the primary factor that accounts for the natural acceleration of the vehicle when traveling downhill.