Explanation:

- Biogas is a renewable energy source produced through the breakdown of organic matter by microorganisms in the absence of oxygen.

- The primary component of biogas is methane, which makes up about 50-75% of the gas.

- Option 1: Methane - It is the main constituent and is responsible for most of the energy content in biogas. Methane burns cleanly with a blue flame and without smoke.

- Option 2: Hydrogen - Though it can be present in trace amounts, it is not a main component of biogas.

- Option 3: Carbon Dioxide - This is the second most common component of biogas but does not contribute to its fuel value.

- Option 4: Hydrogen Sulphide - Typically present in small quantities and is considered an impurity in biogas.