The hydrolysis reaction (with aq. NaOH) that takes place at the slowest rate, among the following, is :

Solution & Explanation

In an aryl chloride the $\mathrm{C}-\mathrm{Cl}$ bond has partial double-bond character due to resonance of the halogen lone pair into the ring, and the carbon is $s{p}^2$ hybridised, holding the halogen tightly. Nucleophilic substitution/hydrolysis therefore requires drastic conditions and is the slowest. Ethyl chloride ($s{p}^3$, $\mathrm{S}{}_{\mathrm{N}}2$), allyl chloride and benzyl chloride (both give resonance-stabilised carbocations / fast $\mathrm{S}{}_{\mathrm{N}}1$) all hydrolyse readily, so the aryl chloride reacts slowest.