Answer: The correct answer is that they are formed below Earth's surface.
Step-by-step explanation:
Plutons are defined as the intrusive igneous rocks named as plutonic rocks which are formed when the magma gets cools down slowly and solidifies below the Earth's surface.
It is different from volcanic rocks which are formed when lava gets cools down and solidifies on the Earth's surface.
Hence, the correct answer is that they are formed below Earth's surface.