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Q.
$S_N1$ reaction is fastest in
Haloalkanes and Haloarenes
Solution:
The $S _{ N } 1$, reaction proceeds in two steps. First step involves slow separation of halogen to form carbonium ion. Second step involves attack of nucleophile on carbonium ion from both sides to form alcohol.
So any species which is capable of forming stable carbonium ion would undergo nuleophilic substitution by $S _{ N } 1$. Benzyl is very stablilised. Hence benzyl chloride follow $S _{ N } 1$ most readily.