The correct option is D Carbon dioxide
Biogas, in India, is primarily obtained from cow dung. Cow dung is rich in methanogens. Methanogens are a type of bacteria which produce large amounts of methane along with carbon dioxide and hydrogen gas. In the production of biogas, the cow dung is converted into a slurry by mixing with water and is allowed to decompose and get digested by anaerobic bacteria. This produces gases such as methane, carbon dioxide and hydrogen. After production of biogas, the leftover solid waste is used as manure, since it is rich in organic nutrients.