The correct option is A oxygen
Methanogens:
Methanogens like Methanobacterium are anaerobic bacteria that can act upon organic matter like cellulose, releasing a mixture of gases composed of hydrogen, carbon dioxide and methane.
Hydrogen sulfide:
Small amount of hydrogen sulfide (H2S) is also produced during methanogenesis.
Oxygen:
Oxygen is not a byproduct of this anaerobic digestion process.
Final answer:a. oxygen