Methanogens are bacteria that produce methane gas. They are commonly found in environments with low oxygen levels, such as the rumen of cattle, anaerobic sludge, and cattle excreta. Methanogens play an important role in the decomposition of organic matter and contribute to the production of methane, a potent greenhouse gas.

Option 1: Rumen of cattle. Methanogens are present in the rumen of cattle. This is because the rumen is an anaerobic environment with low oxygen levels, making it suitable for methanogen activity.

Option 2: Aerobic environment. Methanogens are not typically found in aerobic environments. This is because aerobic environments have high oxygen levels, which are not favorable for the survival and activity of methanogens. Therefore, the presence of methanogens is unlikely in an aerobic environment.

Option 3: Anaerobic sludge. Methanogens are commonly found in anaerobic sludge. Anaerobic sludge is an environment with little to no oxygen, making it suitable for methanogens to thrive and produce methane.

Option 4: Excreta of cattle. Methanogens are present in the excreta of cattle. This is because the excreta provide a suitable