The increasing order of the reactivity of the following halides for...

Rate of ${S}_{N}1$ reaction is directly dependent on the stability of carbocation intermediate.
(II) is primary alkyl halide.
(I) is secondary alkyl halide.
Primary alkyl halide is less reactive towards ${S}_{N}1$ reaction than secondary alkyl halide.
In (III), the carbocation generated is stabilised due to resonance. Hence, (III) is most reactive towards ${S}_{N}1$ reaction.