The ${\mathrm{S}}_{\mathrm{N}}1$ solvolysis of $(\mathrm{CH}{}_3){}_3\mathrm{C}-\mathrm{Cl}$ is fastest in

The rate-determining step forms an ion pair, so the transition state has strong charge separation. Water, with high polarity and strong hydrogen bonding to $\mathrm{Cl}{}^{-}$, stabilises it best. The nonpolar solvents hexane, benzene and $\mathrm{CCl}{}_4$ do not.