The correct answer is option 2: Methane. Methane is a major constituent of biogas. Biogas is a renewable source of energy that is produced through the decomposition of organic matter, such as animal waste, agricultural residues, and sewage. This decomposition process occurs in the absence of oxygen, known as anaerobic digestion.

During anaerobic digestion, microorganisms break down the organic matter and produce a mixture of gases, including methane (CH4) and carbon dioxide (CO2). The composition of biogas can vary depending on the feedstock used and the specific conditions of the digestion process. However, methane typically makes up the largest proportion of biogas, ranging from 50% to 75%.

Methane is a valuable component of biogas as it is a potent greenhouse gas and has a high energy content. It can be captured and used as a fuel for heating, electricity generation, or as a transportation fuel. Methane can be utilized in combined heat and power (CHP) systems, where the heat produced during electricity generation is also harnessed for various purposes, thereby increasing overall energy efficiency.

In summary, methane is a major constituent of biogas and plays a crucial role in harnessing renewable energy from