> Hydrocarbons having double or triple bonds do not burn completely in air and have unburnt carbon which results in blackish smoky flames. This is due to the fact that unsaturated hydrocarbons like C2H2,C6H6, etc have relatively more carbon content than that of saturated hydrocarbons like CH4,C2H6, etc. Let us try to understand it with an example: > Ethane (CH3CH3) has four hydrogen atoms more than that of ethyne (CH≡CH). In other words, the carbon to hydrogen ratio in acetylene is more than that of ethane. More percentage of carbon requires more oxygen for combustion. We can rule out methane (CH4) and ethane (C2H6) from the above given options. > Both ethene (C2H4) and benzene (C6H6) have unsaturation in them. But benzene is an aromatic compound having relatively more carbon content (carbon to hydrogen ratio). So it does not get completely oxidized during combustion and gives out sooty flames.