The burning of fossil fuels releases nitrogen oxides into the atmosphere. Which of the following is NOT a fossil fuel?

Answer: D — Oxygen

Oxygen is correct because it is not a fossil fuel. Fossil fuels, such as diesel, natural gas, and coal, are formed from the remains of ancient plants and animals that have been subjected to heat and pressure over millions of years. They are carbon-based energy sources used for various forms of power generation and transportation. Oxygen, on the other hand, is a chemical element that is essential for combustion but is not a fuel itself. It is produced by natural processes, including photosynthesis, and does not derive from fossilized organic materials.