The correct answer is option 4, Nitrogen dioxide. Biogas is primarily composed of methane (option 3), which is the main component responsible for its flammability. Methane is produced through the anaerobic digestion of organic matter, such as waste from plants, animal manure, and sewage.

Carbon dioxide (option 1) is also present in biogas, although it is not as flammable as methane. Carbon dioxide is produced during the anaerobic digestion process and contributes to the overall energy content of biogas.

Hydrogen (option 2) can be a minor constituent of biogas, but it is not a crucial component. Instead, it is often present in small amounts due to the breakdown of carbohydrates during the anaerobic digestion process.

Nitrogen dioxide (option 4) is not a constituent of biogas. Nitrogen dioxide is a distinct compound that is formed through the reaction of nitrogen and oxygen when fuels are burned at high temperatures. It is an air pollutant and is not naturally produced through the anaerobic digestion process.

In conclusion, while carbon dioxide, methane, and hydrogen are all components of biogas, nitrogen dioxide is not.