Answer: The correct answer is force of gravity.
Step-by-step explanation:
Gravity is the force with which one body attracts the other body.
It goes down a hill with the increase in the kinetic energy. The kinetic energy is due to the motion of a wagon.
At the hill the gravity is lesser in comparison to the gravity on the surface of the earth.
When a wagon rolls down a hill it speeds up. Here, the earth attracts the wagon with the force of gravity. It accelerates down a hill with an acceleration due to gravity.