Answer: option A. Gravity
As per plate tectonics theory therer are three forces that cause plate motion: basal drag, ridge push and slab pull.
Basal drag is caused by convection currents in the mantle.
Ridge push is the name for the force due to the fact that the mid-ocen ridges are higher (more elevated) than the seafloor, which causes that the surrounding rocks are pulled down and away from the ridge pushing the plates.
Slab pull refers to the fact than when a slab sinks it pulls on the rest of the plate.
