The primary bromide shown reacts with $\mathrm{NaI}$ in acetone. Which of the structures A to D reacts most slowly with $\mathrm{NaI}$ in acetone under identical conditions (assume only $\mathrm{S}{}_{\mathrm{N}}2$ is possible)?

The reference is ethyl bromide. Relative $\mathrm{S}{}_{\mathrm{N}}2$ rates fall with branching: allyl bromide (activated) > propyl bromide > cyclopentyl bromide (secondary) >> neopentyl bromide, where the quaternary beta carbon almost completely blocks backside attack. The slowest is neopentyl bromide, Structure C.