a
Since \(S_N1\) reactions involve the formation of carbocation as intermediate in the rate determining higher will be the reactivity of alkyl halides toward \(S_N1\) route. Now we know that stability of carbocations follows the order \(:\,3^o>2^o>1^o,\) so \(S_N1\) reactivity should also follow the same order.
\(3^o > 2^o > 1^o >\) Methyl (\(S_N1\) reactivity)
