d. all of the above
Fossil fuel is a word used to define a gathering of sources of energy produced by natural methods like anaerobic disintegration during the Carboniferous age millions of years ago. During the Carboniferous age, the planet was incorporated with swamps abounding with plants and organisms. When these plants and animals died, they sunk to the depths of these marshes and oceans.
The benefits of fossil fuels are that they are plentiful and available, they contribute a large amount of concentrated power, they are comparatively low cost and they can be carried relatively easily.