- Once means one time
- If an organism is dead then that organism was living at one time
- Therefore, if an organism is dead then that organism is once-living

- If an organism is dead then that organism is once-living
- Fossil fuels / fossils are formed from dead organisms over time
- Therefore, fossil fuels are formed from once-living organisms over time

- To produce something can mean to form something
- A source of something emits / produces / generates / provides that something
- Hence, a source of something forms that something

- Fossil fuels are formed from once-living organisms over time
- A source of something forms that something
- So, once-living organisms are a source of fossil fuels

So, once-living organisms are a source of fossil fuels
[Question]What is the source of fuels such as coal and natural gas?
[Context]- To produce something can mean to form something 
- A source of something emits / produces / generates / provides that something 
- Fossil fuels / fossils are formed from dead organisms over time 
- Once means one time 
- If an organism is dead then that organism was living at one time