The organism present in the rumen (first stomach) of cattle is:

The correct answer is Option (1) → Methanobacterium

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

(1) Methanobacterium — Correct. This methanogen lives in the rumen of cattle.

(2) Trichoderma — Incorrect. Trichoderma is a free-living fungus of the root region. It is used as a biocontrol agent against plant diseases.

(3) Rhizobium — Incorrect. Rhizobium lives in the root nodules of leguminous plants and fixes atmospheric nitrogen.

(4) Glomus — Incorrect. Glomus is a fungus that forms mycorrhiza with plant roots and helps them absorb phosphorus.

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.”