b
Secondary, tertiary and benzylic halides appear to react by a mechanism that involves the formation of a carbocation and proceeds through \(\mathrm{S}_{\mathrm{N}} 1\) mechanism which takes place in two steps. The first step is the slow step \(-\) it is the rate-determining step. In this step, a molecule of alkyI halide ionizes and becomes an alkyl cation. Therefore, greater the stability of carbocation greater will be its tendency to react via \(S_{N} 1\) mechanism.

\(\mathrm{R}-\mathrm{X} \rightleftharpoons \mathrm{R}^{+}+\mathrm{X}^{-}\)

In the second step, the intermediate alkyl cation reacts rapidly with water to produce alcohol.

\(\mathrm{R}^{+}+\mathrm{H}_{2} \mathrm{O} \rightleftharpoons \mathrm{ROH}\)

Hence, the order of reactivity of halides for the \(\mathrm{S}_{\mathrm{N}} 1\) mechanism is \(\mathrm{II}<\mathrm{I}<\mathrm{III}\).