The correct answer is B) gravity.
The force that causes a wagon to speed up as it rolls down the hill is called gravity.
Scientists define Gravity as the force by which bodies are drawn to the center of a planet. It is the force of gravity than maintain planets in its orbits around the Sun. All the things that have mass have gravity. For instance, that is why the human can stay on the surface of the earth. If you jump, you return back to the surface of the Earth. If you throw an object, it always falls down. In the case of the question, the force that causes a wagon to speed up as it rolls down the hill is gravity.
