Answer:
Option (D)
Step-by-step explanation:
The motion of tectonic plates is controlled by both the factors, namely the convection current and gravity. Both this factors occurs simultaneously.
The magma when heated becomes less dense (gravity decrease), and slowly rises up towards the surface of the mantle. As the magma reaches the upper part of the mantle, its temperature decreases and density slowly increases and it eventually sinks. This continuous motion of magma from the core mantle boundary to the surface of the mantle forms a current known as the convection current, that forms in the mantle. It is due to the heat energy radiated from the interior of the earth.
This plate motion takes place in 3 ways namely the divergent, convergent and the transform type of plate boundary.
In the divergent plate boundary, due to the greater density, the oceanic plate subducts below the continental plate.
Thus, in all aspects, the plate motion is initiated by both gravity and gravitational force.
Hence, the correct answer is option (D).