Towards iodide ion in acetone (SN2), which order of decreasing reactivity is correct for (a) $\mathrm{CH}{}_3\mathrm{Br}$, (b) $\mathrm{CH}{}_3\mathrm{CH}{}_2\mathrm{CH}{}_2\mathrm{Br}$, (c) $(\mathrm{CH}{}_3){}_2\mathrm{CHCH}{}_2\mathrm{Br}$, (d) $(\mathrm{CH}{}_3){}_3\mathrm{C}-\mathrm{CH}{}_2\mathrm{Br}$?

In SN2 the nucleophile attacks the back side of the carbon bearing the halogen, and crowding at that carbon or at the adjacent beta carbon slows the reaction. Methyl is the least hindered, then the unbranched propyl, then the beta-branched isobutyl, and the extremely crowded neopentyl is the slowest.