The right option is; b. gravity
Mass movement is caused by gravity.
Mass movement is the downward movement of large amounts of surface materials such as rock or soil which is caused by the influence of gravity. Mass movement can occur very slowly or suddenly and it is caused mainly by gravity along with other geological factors such as water, wind, earthquakes, and lack of vegetation. Types of mass movements are flows (such as debris flows, mudflows), slumps, and rock slides.