The correct option is A Archaebacteria
Methanogens are group of bacteria belonging to archaebacteria. They are found in the gut of ruminants and are known to produce methane as an end product of their anaerobic respiration. The dung of these animals is used in biogas plants for the production of biogas.
Cyanobacteria help in nitrogen fixation. Mycoplasma are infectious in nature and do not produce biogas.