Which one is the major gas present in biogas?

The correct answer is Option (3) → CH₄

Biogas is a fuel made by microbes. Its main gas is methane (CH₄), which is what makes biogas burn. The bacteria that make it are called methanogens; they grow without oxygen on materials such as cattle dung.

NCERT Extract:
Microbes in Production of Biogas – “Biogas is a mixture of gases (containing predominantly methane) produced by the microbial activity and which may be used as fuel. … 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.”