Answer: A). Methanogen
Methanogens are anaerobic unicellular microorganisms. They belong to the domain archea. They are found in anerobic environments like swamps, bogs, sewage sludge, marshes and also in the rumen part of the intestines of the cattle. They use hydrogen, carbon dioxide, acetate, formate and methanol to produce methane and heat as a result of anaerobic fermentation. Their main function is the removal of excess of hydrogen and the fermentation products. The release of the methane to the atmosphere contributes to the carbon cycle.