Each of the following bromides is treated separately with $\mathrm{NaI}$ in acetone under identical conditions. Which one reacts fastest by the SN2 pathway?

(Bromomethyl)cyclohexane is a primary halide, and the beta-branching from the ring is modest. Bromocyclohexane is secondary and hindered. 1-Bromo-1-methylcyclohexane is tertiary and does not undergo SN2. Bromocyclopropane is very slow because the SN2 transition state requires a planar, sp2-like carbon with 120 degree angles, which adds large angle strain to the three-membered ring.