The correct option is
C Methanogens
Marsh gas is also called as biogas. It is also called as methane gas as it primarily consists of methane gas along with hydrogen sulfide and carbon dioxide in minor composition. It is a produced by the decomposition of vegetable matter under water by anaerobic bacteria. The marsh gas is produced by Methanogens that are found in marshes.
Thus, the correct answer is 'Methanogens.'