Biogas typically refers to a mixture of different gases produced by the breakdown of organic matter in the absence of oxygen. Biogas can be produced from raw materials such as agricultural waste, manure, municipal waste, plant material, sewage, green waste or food waste. Biogas is a renewable energy source.

Introduction:
Fossil fuels are hydrocarbon-based resources that are formed from the remains of plants and animals that lived millions of years ago. They are considered non-renewable resources and are used as a source of energy. The question asks which one of the given options is not a fossil fuel.

Explanation:
Among the given options (a, b, c, and d), option 'D' - Biogas is not a fossil fuel. Here's why:

1. Liquified Petroleum Gas (LPG):
- LPG is a flammable hydrocarbon gas that is obtained from natural gas processing and petroleum refining.
- It consists mainly of propane and butane, which are extracted from crude oil and natural gas deposits.
- LPG is widely used as a fuel for heating, cooking, and transportation.

2. Coal Gas:
- Coal gas is a flammable gas that is produced during the destructive distillation of coal.
- It is mainly composed of methane, hydrogen, carbon monoxide, and other volatile gases.
- Historically, coal gas was used for lighting, heating, and cooking, but it has been largely replaced by natural gas in most countries.

3. Compressed Natural Gas (CNG):
- CNG is a gaseous fuel that is primarily composed of methane (CH4).
- It is extracted from natural gas wells or produced as a byproduct of oil refining.
- CNG is used as a cleaner alternative to gasoline and diesel for vehicles, as it emits fewer pollutants.

4. Biogas:
- Biogas is a renewable source of energy produced from organic matter such as animal manure, crop residues, and food waste through a process called anaerobic digestion.
- It consists mainly of methane (CH4) and carbon dioxide (CO2), with small amounts of other gases.
- Biogas can be used for heating, electricity generation, and as a vehicle fuel.

Conclusion:
Based on the explanation above, option 'D' - Biogas is not a fossil fuel. Unlike the other options, biogas is produced from organic matter through a biological process, while fossil fuels are formed from ancient organic material over millions of years.