Which bacterium is typically found in the rumen of cattle?

The correct answer is Option (3) → Methanobacterium

Some bacteria grow without oxygen on cellulose-rich material and give off large amounts of methane along with CO₂ and H₂. They are called methanogens, and a common one is Methanobacterium. These bacteria live in the rumen (a part of the stomach) of cattle, where they help break down the cellulose in the cattle’s food. For the same reason, cattle dung (gobar) is rich in them and is used to make biogas.

NCERT Extract:
Microbes in Production of Biogas – “… 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.”