The stability of earth materials on slopes depends on the balance of gravitational and frictional forces. which of the following does not have a measurable effect on the balance between gravity and friction?
a. type of rock or sediment that is making up the slope
b. presence of water
c. types of vegetation on the slope
d. atmospheric air pressure
e. steepness of a slope

c. types of vegetation on the slope

Explanation:

Friction and gravity plays a major role on a slope. Usually the higher the friction the likely the rocks will stay. The steeper the slope the higher the gravity and the likely the sediment will slide.

Water plays a major role in balancing friction and gravity because the more the water on the surface of the rocks or sediment the likely the rocks and sediment slides as gravity is increased. The kind of sediment  or rocks also plays a  major role as rocks like clay and mud discourage water seeping through it, water are retained on it surface thereby increasing gravity.

climates changes brings about changes in atmospheric air pressure that causes increase in water contents.

The vegetation on the slope is unlikely to affect friction and gravity.  

answergeosphere;