The correct option is C. Scientists believed that the earth tectonic plates are in constant motion, this motion is slow and its rates vary. The motions of the tectonic plates are caused by the convection current which is found in semi molten mantle. The convection current are created by heat within the earth surface. When a semi molten rock is heated, it become less in density compared to its surrounding and thus rises up. When the less dense molten rock reaches the crust above, it spread out and carries the tectonic plate above with it. When the molten magma cools down, it sinks and the whole process is repeated leading to continuous movement of the tectonic plates.
