Answer:
The correct answer option is 'gravity acting on the edges of plates and convection in the mantle'.
Step-by-step explanation:
The two processes which the scientists think that move the Earth's lithosphere plates are:
1) gravity acting on the edges of the plates; and
2) convection in the mantle
The gravitational body force pulling the slab downwards is balanced by the resistive forces acting on the plate while mantle convection is the main way through which the heat from Earth's interior is transferred to its surface.