Answer:
a. The convective circulation in the upper mantle
Step-by-step explanation:
The engine that powers plate movement is the formation of convective cells within the mantle.
Convection is a heat transfer method usually that occurs within fluids.
The internal heat engine provides a source of temperature that makes part of the mantle to occur in a weak molten form. This is the asthenosphere. Overriding the layer is the lithosphere.
Convection allows for dense and cold lithospheric plate to sink into the asthenosphere and the hot, buoyant asthenosphere rises to the surface to replace it. The alternation between these processes results in mantle convection and eventual plate movement.