Blank is the direct movement of materials by gravity

a. mass movement
b. creep
c. erosion
d. gravitational attraction
e. weathering

"Mass Movement" is the direct movement of materials by the effect of gravity.

Option (a);

EXPLANATION:

"Mass Movement"  typically occurs along the hillsides and mountains. It means the "movement of soil, rock, mud or snow" down the slope due to gravity. The main reason for this movement is the "angle of the slope" of the hill. If it is "steep in angle" the gravity pulls the materials in downside causes mass movement.The "angle of the slope" used to determine the fast movement of the materials in the downside direction and it happens either very quickly or very slowly. "Mass movement" is also called "mass wasting".