The correct answer is Animal dung.

Biogas: 

• It is the mixture of gases produced by the breakdown of organic matter in the absence of oxygen (anaerobically).
• It primarily consists of Methane and Carbon dioxide. 
• It can be produced from raw materials such as agricultural waste, animal manure, municipal waste, plant material, sewage, green waste, or food waste.
• It is a renewable energy source.
• In India, it is also known as "Gobar Gas".
• The gases methane, hydrogen, and carbon monoxide (CO) can be combusted or oxidized with oxygen.
• This energy release allows biogas to be used as a fuel.
• It can be used for any heating purposes, such as cooking.
• It can also be used in a gas engine to convert the energy in the gas into electricity and heat.

About coal:​

• Coal is a sedimentary black or dark brown rock that varies in carbon composition.
• As coal contains mainly carbon, the slow process of conversion of dead vegetation into coal is called carbonisation.
• Since it was formed from the remains of vegetation, coal is also called a fossil fuel.
• Coal is one of the fuels used to cook food.
• Earlier, it was used in railway engines to produce steam to run the engine.
• It is also used in thermal power plants to produce electricity.
• Coal is also used as a fuel in various industries.