Concept:Bromine water is decolourised by compounds that can undergo addition reactions with bromine.
Explanation:Hexane and pentane are alkanes, so they normally do not react with bromine water.
Benzene is an aromatic compound and does not easily decolourise bromine water.
Cyclobutane is a small-ring cycloalkane with ring strain.
It can undergo an addition reaction with bromine, breaking its ring to form a dibromo compound.
This addition removes the reddish-brown colour of bromine water, causing decolourisation.
Hence, among the given options, cyclobutane will decolourise bromine water.
Answer:B. Cyclobutane