In which one of the following, methano- gens, i.e., methane producing bactcria arc not present ?

Methanogens are a specialized group of microorganisms, specifically archaea, that produce methane as a metabolic byproduct. They are strictly obligate anaerobes, meaning they are highly sensitive to oxygen and cannot survive or function in aerobic environments [7]. These organisms thrive in oxygen-free habitats where they decompose organic matter. Common locations for methanogens include the rumen of cattle, where they assist in digestion [4], and anaerobic sludge during wastewater treatment [6]. They are also found in the excreta (dung) of cattle, where they continue the decomposition process in anaerobic conditions [4]. Because oxygen is toxic to them, they are fundamentally absent from aerobic environments [7]. Their unique metabolic pathway, methanogenesis, is inhibited by the presence of oxygen, making an aerobic environment the only setting among the choices where they would not be present [5].

Sources

1. [1] https://pmc.ncbi.nlm.nih.gov/articles/PMC2765141/
2. [7] https://www.sciencedirect.com/science/article/pii/S2665906924000047
3. [4] Environment and Ecology, Majid Hussain (Access publishing 3rd ed.) > Chapter 7: Climate Change > 2. greenhouse gases > p. 11
4. [6] https://pmc.ncbi.nlm.nih.gov/articles/PMC6488025/
5. [3] https://pmc.ncbi.nlm.nih.gov/articles/PMC8973982/
6. [5] Environment, Shankar IAS Acedemy .(ed 10th) > Chapter 17: Climate Change > Human induced: > p. 256