(a) Transition metals are able to form complex compounds due to the small size of metal, high ionic charge and availability of vacant $d$-orbital.(b) $E^{\circ}$ values of $(\mathrm{Mn}^{2+}/\mathrm{Mn})$ and $(\mathrm{Zn}^{2+}/\mathrm{Zn})$ are more negative than expected due to the greater stability of half filled $d$-subshell of $\mathrm{Mn}^{2+} (3d^5)$ and completely filled $d$-subshell of $\mathrm{Zn}^{2+} (3d^{10})$.(c) Due to a very small energy gap between $5f, 6d$ and $7s$-subshells resulting in easier excitation of the outermost electrons to higher energy levels.