Which of the following compounds undergoes the fastest ${\text{S}}_{\text{N}}1$ substitution reaction?

An ${\text{S}}_{\text{N}}1$ reaction proceeds via the formation of a carbocation intermediate, and its rate is directly governed by the relative thermodynamic stability of this carbocation.
When $\text{MeO}-{\text{CH}}_2-\text{Cl}$ dissociates, it generates the methoxymethyl carbocation:

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

This carbocation is extraordinarily stabilized due to strong resonance donation from the lone pair of electrons on the adjacent oxygen atom (forming a stable oxocarbocation structure where all atoms have completed octets). This makes it highly reactive under ${\text{S}}_{\text{N}}1$ conditions compared to regular primary or destabilized carbonyl alkyl halides.