Here's a problem to solve: Which of the following changes would end Earth's tectonic activity?

Among the 4 following options, which is the correct answer?

- A: Earth's crust becomes a cooler solid.
 
- B: Earth's mantle becomes a warmer semiliquid.
 
- C: Earth's mantle becomes cooler and solid.
 
- D: Earth's outer core becomes warmer.
 