The force ultimately responsible for mass movement a. erosion c. gravity b. deposition d. weathering

The right option is; c. gravity

Gravity is the force ultimately responsible for mass movement  

Mass movement, is the bulk movements of soil, sand, regolith, and rock debris in a predominantly vertical direction in response to the force of gravity without the aid of a transporting medium such as wind, water, or ice. Gravity is the force ultimately responsible for mass movement. Gravity is the force that pulls all things that has mass or energy towards one another and toward the center of the earth.