The fastest ${\text{S}}_{\text{N}}1$ reaction occurs with which of the following compounds?

The rate of an ${\text{S}}_{\text{N}}1$ reaction depends on the stability of the carbocation intermediate formed after the leaving group (${\text{Cl}}^{-}$) departs.
* For $\text{MeO}-{\text{CH}}_2-\text{Cl}$, loss of ${\text{Cl}}^{-}$ generates the methoxymethyl cation ($\text{MeO}-{\text{CH}}_2^{+}$). This carbocation is highly stabilized by resonance via lone-pair donation from the adjacent oxygen atom:

$$\text{Me}-\ddot{\text{O}}-{\text{CH}}_2^{+}⟷\text{Me}-{\text{O}}^{+}={\text{CH}}_2$$

Because every atom in this resonance structure satisfies the octet rule, this intermediate is exceptionally stable, allowing the reaction to proceed exceptionally fast.