Methanobacterium are anaerobic organisms. Biogas refers to a mixture of different gases produced by the breakdown of organic matter in the absence of oxygen. Activated sludge-sediment in settlement tanks of sewage treatment plant is a rich source of aerobic bacteria. Biogas can be produced by anaerobic digestion with anaerobic bacteria. Thus, the correct answer is option (D), 'Activated sludge-sediment in settlement tanks of sewage treatment plant is a rich source of aerobic bacteria'.