Answer:
The correct answer is option D. Gravity.
Explanation:
Mass movement is mainly caused by the force of gravity.
We can find this kind of force in all the parts that make up the earth's surface.
If an object is on a flat surface, which is on par with the surface of the Earth, the force of gravity will "pull it down." This is what holds us to the ground and prevents us from gravity affecting us. As long as we remain on the flat surface, we will not move down. But if somehow the surface where we are standing weakens or fails, the support mass will fall down.
