Which of the following alkyl bromides reacts fastest with $\mathrm{NaI}$ in dry acetone by the ${\mathrm{S}}_{\mathrm{N}}2$ mechanism?

SN2 rate falls with crowding at the reacting carbon: methyl $>$ primary $>$ secondary $>$ tertiary. Methyl bromide has only three hydrogens around the carbon, so backside attack is easiest.