Explain how fossil fuels found in the earth’s crust are ultimately the result of biological processes.

Fossil fuels are hydrocarbons, primarily coal, fuel oil or natural gas, formed from the remains of dead plants and animals. Fossil fuel is a general term for buried combustible geologic deposits of organic materials, formed from decayed plants and animals that have been converted to crude oil, coal, natural gas, or heavy oils by exposure to heat and pressure in the earth's crust over hundreds of millions of years by the result of biological processes. The organic compounds break down and convert into other compounds via biological processes.

Fossil fuels are made up of organic matter  which were acted upon by microorganism. Thus they can be considered as product resulting from biological process.

Explanation:

Fossil fuels are made up of dead organic matter that got buried deep inside the earth. With the increase in depth of burial, the temperature resulting from geothermal gradient increase and thus the dead organic matter are converted into hydrocarbon resources.  

In the early stage of fossil fuel formation , anaerobic bacteria act upon the dead organic matter to produce methane. This methane gets entrapped with in the reservoirs to form source of natural gas and the remaining organic waste are further converted into coal , sedimentary fossil reserves etc.  

Since microorganisms are involved in the production of fossil fuels, it can be said that fossil fuels found in the Earth’s crust are ultimately the result of biological processes