The answer is: C. They take millions of years to form.
Fossil fuel is a fuel formed by anaerobic decomposition of buried dead organisms.
If there was oxygen the organic matter would not decomposed and formed oil and gas, because there would be reaction of oxidation.
The age of the organisms and their resulting fossil fuels is typically millions of years
Fossil fuel is mixture (solution) consist of several different hydrocarbons (alkanes, alkenes...), aromatic compounds.