Methyl tert-butyl ether, $\mathrm{CH}{}_3-\mathrm{O}-\mathrm{C}(\mathrm{CH}{}_3){}_3$, is to be made in good yield by an SN2 Williamson synthesis. The correct pair of reactants is

Williamson synthesis works by SN2 attack of an alkoxide on a halide, so the halide must be methyl or primary. A tertiary halide with a strong alkoxide undergoes E2 to give isobutene. The bulky alkoxide $(\mathrm{CH}{}_3){}_3\mathrm{CO}{}^{-}$ therefore attacks unhindered $\mathrm{CH}{}_3\mathrm{I}$.