The correct answer is - radioactive decay inside earth.
The geothermal energy is a natural renewable type of energy. It is caused by the radioactive decay and the continual heat loss from the Earth's formation. This a heat that comes out of the core-mantle boundary and this temperature can go up to 4000 C degrees, which even though cools off significantly when it comes on the surface it still is very hot and is propelled by very high pressure. This type of energy is found on places close to the volcanoes because these are the places where the crust is cracked and is exposed to the heat and pressure from below.