The correct answer is 3 Asthenosphere.
The radioactive decay of elements deep in the interior of the Earth creates magma in the asthenosphere which is the part of the mantle, the middle sphere of the Earth. Large convection currents in the asthenosphere transfer heat to the surface, where long soft magma break apart the plates at the spreading centers, creating divergent plate boundaries. Thus, convection currents in the magma cause plate movement.
