Which one of the following compound is most reactive towards ${\mathrm{SN}}_2$ reactions ?

Rate of ${\mathrm{SN}}_2$  reactivity $\propto$ leaving group ability;

Leaving group ability $\propto \frac{1}{\mathrm{Basic} \mathrm{strength}}$;

Basic strength of  ${\mathrm{I}}^{-}<{\mathrm{Br}}^{-}<{\mathrm{Cl}}^{-}<{\mathrm{F}}^{-}$;

Leaving gp ability will have reverse order;;