What is the “reserve” of a fossil fuel?

The correct answer is underneath the Earth's surface or in the crust layer of Earth.

Explanation:

Fossil fuels are the combustible organic materials like coal, natural gas,  crude oil which are formed from the deposits of the decayed parts of the animals and plants which died millions of years ago while reserves are the deposits of these natural resources like minerals, elements and fuel.

Fossil fuels are usually found underneath the Earth's surface or in the crust layer of Earth at varying depths.  

Thus, underneath the Earth's surface or in the crust layer of Earth is the correct answer.

evidence and logical support is essential for a scientist to do in order to maintain credibility. this is because no one can believe without any logic and without any evidence.hence a scientist can maintain credibility by proving himself and by answering logically so experiments prove very beneficial. logical hypothesis give conclusion which is acceptable. therefore, there are two main factors to maintain credibility by a scientist is evidence and logical support.