What natural force causes a vehicle to increase speed going downhill?

Answer: C — Gravity

Gravity is the force that causes objects to accelerate towards the Earth's surface. When a vehicle is travelling downhill, the force of gravity is acting on the vehicle and causing it to increase in speed. The other options, such as wind resistance, inertia, and friction, do not directly impact the speed of a vehicle going downhill. Wind resistance can create drag, inertia is the tendency of an object to resist changes in its state of motion, and friction works against the motion of an object, so none of these forces would cause a vehicle to increase speed while going downhill.