What process in Earth's mantle
do scientists believe is driving
the movement of the plates?

It's convention currents

Explanation:

in cartography, a contour line (often just called a "contour") joins points of equal elevation (height) above a given level, such as mean sea level. a contour map is a map illustrated with contour lines, for example a topographic map, which thus shows valleys and hills, and the steepness or gentleness of slopes.

explanation:

they are all solids.