Methanogens, commonly found in the sludge during sewage treatment are:

The correct answer is Option (1) → Anaerobic

Methanogens are bacteria that grow anaerobically on cellulosic material and produce methane along with carbon dioxide and hydrogen. One common example is Methanobacterium. They are found in the anaerobic sludge of sewage treatment plants and in the rumen of cattle. They are the organisms that make biogas.

(1) Anaerobic — Correct. Methanogens grow without oxygen, in the anaerobic sludge.

(2) Aerobic — Incorrect. Aerobic microbes need oxygen. In sewage treatment they grow in the aeration tank, where they form flocs, and not in the anaerobic sludge digester.

(3) Facultative aerobes — Incorrect. These can grow with or without oxygen. NCERT describes methanogens as bacteria that grow anaerobically.

(4) Opportunistic aerobes — Incorrect. Aerobes need oxygen. This is not a description of the methanogens.

NCERT Extract:
Microbes in Production of Biogas – “… However, certain bacteria, which grow anaerobically on cellulosic material, produce large amount of methane along with CO₂ and H₂. These bacteria are collectively called methanogens, and one such common bacterium is Methanobacterium. These bacteria are commonly found in the anaerobic sludge during sewage treatment.”