The following gases decolorize bromine water except

Bromine water is a reddish-brown solution of bromine in water. When an unsaturated hydrocarbon gas (C₂H₄, C₂H₂, C₃H₄, C₃H₆) is passed through the bromine water, the double or triple bond of the hydrocarbon is broken, and the bromine is added across the double or triple bond. This results in the decolorization of the bromine water. On the other hand, when a saturated hydrocarbon gas such as ethane (C₂H₆) is passed through bromine water, there is no double or triple bond to break, and therefore no bromine is added to the molecule. Thus, bromine water is not decolorized by ethane. Therefore, the correct answer to the question is C₂H₆.