The composition of biogas is -

The correct answer is Option (3) → \( CH_4, H_2S \) and \( CO_2 \)

Biogas is made when anaerobic bacteria (methanogens) digest organic matter, in sludge digesters or biogas plants. It is mostly methane, with hydrogen sulphide and carbon dioxide; methane makes it burn.

Option (1) \( CH_4, H_2S \) and \( N_2 \) — Incorrect. Nitrogen is not a product of this digestion; the third gas is carbon dioxide.

Option (2) \( CH_4, H_2 \) and \( O_2 \) — Incorrect. The digestion is anaerobic, so no oxygen is given out; and the sulphur gas is missing.

Option (3) \( CH_4, H_2S \) and \( CO_2 \) — Correct. Bacteria in the digesters produce methane, hydrogen sulphide and carbon dioxide, which together form biogas.

Option (4) \( CH_4, SO_2 \) and \( CO_2 \) — Incorrect. Sulphur dioxide forms when sulphur-containing fuels burn in air. Bacteria working without oxygen give hydrogen sulphide instead.

NCERT Extract:
Microbes in Sewage Treatment – …