The correct answer is D. Methane.
Methane is the main component of biogas, which is produced by the anaerobic digestion of organic matter. Anaerobic digestion is a process that occurs in the absence of oxygen, and it is used to treat wastewater, sludge, and other organic wastes. The gas produced by anaerobic digestion is a mixture of methane, carbon dioxide, and other gases. Methane is the most abundant gas in biogas, and it can be used as a renewable energy source.
Nitrogen is the second most abundant gas in the atmosphere, and it is also found in wastewater and sludge. However, nitrogen is not a major component of biogas. Carbon dioxide is the third most abundant gas in the atmosphere, and it is also found in wastewater and sludge. Carbon dioxide is a byproduct of anaerobic digestion, and it is not a major component of biogas. Hydrogen sulfide is a toxic gas that is produced by the anaerobic digestion of sulfur-containing compounds. Hydrogen sulfide is not a major component of biogas.
In conclusion, the gas from sludge digestion tank is mainly composed of methane. Methane is a renewable energy source that can be used to generate electricity, heat, and transportation fuel.