The stench due to leakage from the LPG cylinder is because of:

The correct answer is D, ethyl mercaptan. Pure LPG, which is mostly propane and butane, is odourless. Suppliers add a trace amount of a sulphur-based odorant, a mercaptan, so that a leak can be smelt long before it reaches an explosive concentration. Ethyl mercaptan is the classic odorant for this purpose.

Why the others are wrong:

• Methane (A): It is the main component of natural gas and is also odourless. PNG and CNG are odorised with mercaptan-type compounds for the same reason.
• Propane (B) and Butane (C): These are the fuel components of LPG. They are odourless in their pure form, so they cannot be the source of the smell.

Core concept: odorisation is a safety measure. It works because humans can detect mercaptans at very low concentrations. For Mains, this links to household energy safety, the spread of LPG under PMUY, and the regulatory standards for gas distribution.

Trap: Aspirants assume the smell comes from the gas that is actually burning, namely propane or butane. The odour is added on purpose and is not a property of the fuel.