(a) (i) $5\mathit{f}$ orbital electrons have poor shielding effect than $4\mathit{f}$ orbital electrons.
(ii) Due to $\mathit{d}$ - $\mathit{d}$ transition or the energy of excitation of an electron from lower $\mathit{d}$ orbital to higher $\mathit{d}$-orbital lies in the visible region and presence of unpaired electrons in the $\mathit{d}$-orbital.
(b) $2{\mathrm{MnO}}_4^{-}+6{\mathrm{H}}^{+}+5{\mathrm{NO}}_2^{-}\mathrm{}\mathit{─}\mathit{─}\mathit{─}\mathit{─}▸2{\mathrm{Mn}}^{2+}$ $+3{\mathrm{H}}_2\mathrm{O}+5{\mathrm{NO}}_3^{-}$