(I) (II) (III) Consider the three substituted benzyl chlorides....

The reactivity of alkyl halides towards $S_{N}1$ solvolysis depends on the stability of the carbocation intermediate formed in the rate-determining step.

The carbocations formed from the given substituted benzyl chlorides are:
(I) Benzyl cation ($C_6H_5CH_2^+$)
(II) p-Methoxybenzyl cation ($p-CH_3O-C_6H_4CH_2^+$)
(III) p-Nitrobenzyl cation ($p-NO_2-C_6H_4CH_2^+$)

The stability of these carbocations is influenced by the electronic effects of the substituents on the benzene ring:
1. In compound (II), the $-OCH_3$ group at the para position exerts a strong $+M$ (mesomeric) effect, which donates electron density and highly stabilizes the positive charge on the benzylic carbon.
2. In compound (I), there is no substituent on the ring, so it serves as the reference.
3. In compound (III), the $-NO_2$ group at the para position exerts strong $-M$ and $-I$ (inductive) effects, which withdraw electron density and highly destabilize the carbocation.

Therefore, the order of stability of the carbocations is II > I > III.

Consequently, the decreasing order of reactivity towards $S_{N}1$ solvolysis is II > I > III.

Answer: II > I > III