A gobar gas plant produces:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → Methane
In a gobar gas (biogas) plant, cattle dung mixed with water is broken down by bacteria in the absence of air (anaerobic decomposition) inside a closed digester. The gas formed is mostly methane (roughly 50–75 per cent), with carbon dioxide making up most of the rest. Methane burns with a clean blue flame, which is why the gas is used for cooking and lighting, and the leftover slurry makes good manure.
Why the other options are incorrect
Option (1) Ethane — Incorrect. The bacteria in the digester form methane, the one-carbon hydrocarbon; ethane is not a product of the plant.
Option (2) Hydrogen sulphide — Incorrect. It is present only in traces, as an impurity that gives raw biogas a smell of rotten eggs.
Option (3) Inert gas — Incorrect. Inert (noble) gases such as argon are not formed by decomposition; the gas from the plant is a fuel.