Which energy source is a result of tiny sea creatures being compressed over millions of years?

Oil

Explanation:

Dead plankton falls to the bottom of the ocean and over centuries becomes oil from high pressure and heat

The correct answer is crude oil.

Explanation:

The dead plankton settles at the bottom of the seas and oceans and because of the accumulating layers of sediments gets compressed over millions of years, resulting in the formation of crude oil. This takes place due to heat and high pressure. The extreme pressure and the heat is the source of energy, which leads to compression of tiny sea creatures and results in the formation of natural gas and oil.