Final answer:
The theory that explains Earth's plates being in slow, constant motion is plate tectonics, driven by convection currents in the mantle.
Step-by-step explanation:
The theory that explains Earth's plates being in slow, constant motion is plate tectonics.
Plate tectonics is driven by convection currents in the mantle, which refers to the process of heat escaping from the Earth's interior and causing the mantle material to flow upward and cooler material to sink. This convection creates the driving force behind the movement of Earth's tectonic plates.
The other options listed, such as the gravitational pull from the Moon, solar radiation and heat from the Sun, and magnetic forces within the Earth's core, are not the main driving forces behind plate tectonics.
Therefore, the correct option is C.