(a) Because of availability of partially filled orbitals and comparable energies of ns and (n−1) d-orbitals.
(b) E∘ value for (Zn2+∕Zn) is negative due to stable completely filled d10 configuration in Zn2+. The positive value of (Cu2+∕Cu) accounts for its ability to liberate H2 from acids due to its high enthalpy of atomization and low hydration energy.
(c) Mn can form multiple bonds with oxygen by using 2p-orbital of oxygen and 3d-orbital of Mn because of which it shows highest oxidation state of +7 with fluorine, Mn cannot form multiple bonds thus shows an oxidation state of +4 .