(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 ${\left({\mathrm{CH}}_3\right)}_3\mathrm{CCl}$.