according to the theory of plate tectonics, which forces cause the movement of plates in the earth's crust?

a.
earthquakes and volcanic eruptions

b.
magnetism and energy from solar radiation

c.
convection and gravity

d.
convection and the moon’s gravitational pull

C.  convection and gravity  

Explanation:

The theory of plate tectonic suggest that the Earth's layer is divided into seven major plates which glide over each other. Convection force and gravity is responsible for the movement of these plates.

The molten rock in the mantle rise up due to intense heat produced by Earth's core. As they rise, they eventually cool down and sink again. This convection process continues and leads to movement of plates. The force that causes this motion is the gravity. When one plate rubs with the other plate, earthquake occurs.