In in this reaction a stable tertiary carbocation is possible so ${\mathrm{S}}_{\mathrm{N}}1$ mechanism is used and ${\mathrm{I}}^{-}$ in ${\mathrm{S}}_{\mathrm{N}}1$ is taken with stable carbocation.
In rest other options ${\mathrm{S}}_{\mathrm{N}}2$ mechanism will be used as stable carbocations are not possible hence ${\mathrm{I}}^{-}$ goes with $\mathrm{C}{\mathrm{H}}_3$ group to give $\mathrm{C}{\mathrm{H}}_3\mathrm{I}$ and respective alcohols.