Answer : 4. "Methane"
Explanation :

Answer: D) Methane Explanation: Natural gas is primarily composed of methane, but also contains ethane, propane, and heavier hydrocarbons.   Natural gas is a colorless, tasteless, odorless, and non-toxic gas. Because it is odorless, mercaptan is added to the natural gas, in very small amounts to give the gas a distinctive smell of rotten eggs. This strong smell can alert you of a potential gas leak.   It also contains small amounts of nitrogen, carbon dioxide, hydrogen sulphide and trace amounts of water.