In which of the following, chlorine is least reactive ?

From the option given, in ethyl chloride, allyl chloride and methyl chloride the chlorine is highly reactive as it is bonded to ${\mathrm{sp}}^3$ hybridized carbon atom which can easily undergo nucleophilic substitution reaction.

Chlorobenzene's reduced reactivity is caused by the partial double bond nature of the $\mathrm{C}-\mathrm{Cl}$ bond.

Hence, chlorobenzene is the least reactive.