(a) (i) $\mathrm{C}_6\mathrm{H}_5-\mathrm{CH}_2-\mathrm{Cl}$ (ii) (b) (i) The steric barrier surrounding the target atom affects the rate of the $\mathrm{S}_{\mathrm{N}}2$ reaction. As $\mathrm{CH}_3\mathrm{I}$ presents less of a barrier, it will undergo the $\mathrm{S}_{\mathrm{N}}2$ reaction more quickly than $\mathrm{CH}_3\mathrm{Br}$. (ii) Since $\mathrm{CH}_3\mathrm{Cl}$ is the major halide, it hinders $\mathrm{OH}^-$ stability to act less, allowing it to react with $\mathrm{S}_{\mathrm{N}}2$ more readily than $(\mathrm{CH}_3)_3\mathrm{CCl}$.