Earth's crust becomes a cooler solid. Earth's mantle becomes a warmer semiliquid. Earth's mantle becomes cooler and solid. Earth's outer core becomes warmer.
I gave my students this multiple choice question: Which of the following changes would end Earth's tectonic activity?

Only one answer is correct among these 4 choices:
- Earth's crust becomes a cooler solid.
- Earth's mantle becomes a warmer semiliquid.
- Earth's mantle becomes cooler and solid.
- Earth's outer core becomes warmer.

Could you tell me which one is correct?

Earth's mantle becomes cooler and solid.