Biogas is a type of gas that is produced from organic matter such as animal waste, sewage, and plant material through a process called anaerobic digestion. It is primarily composed of methane.

Methane, which has the chemical formula CH4, is the main component of natural gas and is also the primary component of biogas. It is a colorless and odorless gas that is highly flammable. Methane is produced during the decomposition of organic matter by the action of bacteria in anaerobic conditions.

On the other hand, ethane, butane, and carbon dioxide are not the main components of biogas. Ethane (C2H6) is a hydrocarbon gas commonly found in natural gas but is not a significant component of biogas. Butane (C4H10) is a heavier hydrocarbon gas that is used as a fuel, but it is not commonly present in biogas. Carbon dioxide (CO2) is a byproduct of biogas production and is present in minor amounts, but it is not the primary component.

In conclusion, the correct answer is option 1: Methane is the main component of biogas.