Williamson ether synthesis is the reaction in which an alkyl halide is allowed to react with alkoxide to give ether. The reaction involves ${\mathrm{S}}_{\mathrm{N}}2$ attack of an alkoxide ion on primary alkyl halide. In option $\left(\mathrm{a}\right)$ and $\left(\mathrm{b}\right)$ there are alkoxide and alkyl halide, but option $\left(\mathrm{b}\right)$ is preferred because $\mathrm{I}$ is better leaving group than ${\mathrm{Cl}}^{-}$