The well known insecticide, Gammaxene is one of the stereoisomers of hexachlorocyclohexane. The reagent useful for conversion of benzene into hexachlorocyclohexane is -

The correct option is D $3C{l}_2/hv$
Halogenation of aromatic compound is an addition reaction. In this reactions halogens gets added to the double bonds in aromatic compounds in presence of UV light.

Under ultraviolet light, three molecules of chlorine will react with benzene to produce benzene hexachloride
Benzenehexachloride $\left(BHC\right),C_6{H}_{6}C{l}_6$ is also called Gammaxene and it has been used both as an agricultural insecticide but now it is banned because exposure to BHC can cause acute poisioning and may affect liver functioning in humans.