Assertion: Dung can be used for generation of biogas, commonly called gobar gas.
Reason: Methanobacterium are commonly found in the aerobic sludge during sewage treatment. .

Assertion (A) is true but Reason(R) is not correct.

The correct answer is Option (3) → Assertion (A) is true but Reason(R) is not correct.

Biogas is a mixture of gases (containing predominantly methane) produced by the microbial activity and which may be used as fuel. The type of the gas produced depends upon the microbes and the organic substrates they utilise. 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.