The correct answer is C. Sulphur compound.
LPG (liquefied petroleum gas) is a mixture of propane and butane. It is a colourless, odourless gas that is stored under pressure in cylinders. When LPG leaks, it can be detected by the smell of ethyl mercaptan, which is added to LPG to make it smell. Ethyl mercaptan is a sulphur compound.
Nitrogen peroxide is a colourless, unstable gas that is used as a bleaching agent and disinfectant. Carbon monoxide is a colourless, odourless gas that is produced when carbon is burned in an incomplete combustion. Carbon dioxide is a colourless, odourless gas that is produced when carbon is burned in a complete combustion.
The smell of ethyl mercaptan is very strong and can be detected at very low concentrations. This is why it is added to LPG to make it easier to detect leaks. If you smell the smell of ethyl mercaptan, it is important to evacuate the area immediately and call the fire department.