What is the primary driving force behind the motion of tectonic plates?

The primary driving force behind the motion of tectonic plates is convection currents in the mantle. These convection currents are processes within the Earth's mantle where heat from the core causes the mantle material to rise and fall due to changes in temperature and density. Hot material rises toward the surface, cools down, and then sinks back down to be reheated, creating a continuous cycle. This movement in the mantle exerts a drag force on the base of the tectonic plates, which sit atop the mantle, leading to their movement.

This mechanism is fundamental to the theory of plate tectonics, as it explains how large sections of the Earth's lithosphere can move and interact, leading to geological features such as mountains, earthquakes, and volcanoes. The other choices, such as gravitational pull, magnetic forces, and human activities, do not significantly contribute to the primary movement of tectonic plates. While gravity plays a role in the overall stability of tectonic features, it is the convection currents in the mantle that are the primary movers in the large-scale dynamics of plate tectonics.