Methanobacterium is responsible for:A. Breakdown of celluloseB. Break…

The correct answer is Option (3) → A and C only

Methanogens are bacteria that grow without oxygen (anaerobically) on cellulosic material. They produce a large amount of methane along with CO₂ and H₂. Methanobacterium is a common methanogen.

A. Breakdown of cellulose — Correct. Methanobacterium lives in the rumen (a part of the stomach) of cattle. There it helps in the breakdown of cellulose, which is important for the nutrition of cattle.

B. Breakdown of starch — Incorrect. Methanogens grow on cellulose, not starch.

C. Production of biogas — Correct. Biogas is mainly methane. Cattle dung is rich in these bacteria, so it is used in biogas plants, where the bacteria produce biogas.

D. Biological control of pests — Incorrect. Biocontrol uses other organisms, such as the bacterium Bacillus thuringiensis, the fungus Trichoderma and ladybird beetles. Methanobacterium is not a biocontrol agent.

NCERT Extract:
Microbes in Production of Biogas – “… In the examples cited in relation to fermentation of dough, cheese making and production of beverages, the main gas produced was CO2.. 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. These bacteria are also present in the rumen (a part of stomach) of cattle. A lot of cellulosic material present in the food of cattle is also present in the rumen. In rumen, these bacteria help in the breakdown of cellulose and play an important role in the nutrition of cattle. … Thus, the excreta (dung) of cattle, commonly called gobar, is rich in these bacteria. Dung can be used for generation of biogas, commonly called gobar gas.”