Solution:concept - Biogas is a mixture of gases liberated by the breakdown of organic matter in absence of oxygen.
Biogas is also sometimes called swamp gas, sewer gas, compost gas, and marsh gas in the US. Biogas is a naturally occurring and renewable energy source produced by the decomposition of organic matter by certain bacteria under anaerobic conditions.
It is a mixture of 50-75 % methane ( ), 25-50 % hydrogen ( ) and 2-8% carbon dioxide ( ).
Therefore, the main constituent of biogas is methane.
Hence, the correct answer is 1.