Which of these conditions would likely cause the tectonic plates of Earth to stop moving?

The correct option is;

A. The Earth's crust and mantle reach thermal equilibrium

Explanation:

The movement of the tectonic plates in the theory of plate tectonics is believed to be due to the heating of the mantle from the Earth's core causing convection currents which causes movement of molten rock in the Earth's mantle.

The convection current movement caused by the rising of the heated mantle material close to the Earth's crust and the sinking of cooled material back towards the core is similar to a stove boiling water in a pot.

Therefore, when the Earth's crust and mantle are at thermal equilibrium, there will be no convection current and the tectonic plates of Earth will stop moving.