Wegener attempted to explain the movement of continents but could not identify a source of force strong enough. what did scientists eventually identify as the primary energy source that drives the movement of earth's plates?
a)convection
b)friction
c)gravity
d)volcanoes

Primary energy source that drives the movement of earth’s plates is convection.  

Explanation:

The outermost layer of earth namely the crust is made up of several tectonic plates and these plates are in continuous motion. Phenomena like sea floor spreading are proofs of plate movement. It is this plate movement that causes earthquakes and volcanic eruptions.  

The interior of earth has very high temperature and this high temperature heats up the rocks near mantle, the middle layer of earth. This hot magma rises up the surface and induces the movement of tectonic plates. This cycle is not a quick process but a  slow and gradual one.  

c is the correct answer hope this !