It is because of inability of ns2 electrons of the valence shell to participate in bonding that
(a) Sn2+ is oxidising while Pb4+ is reducing
(b) Sn2+ and Pb2+ are both oxidising and reducing
(c) Sn4+ is reducing while Pb4+ is oxidising
(d) Sn2+ is reducing while Pb4+ is oxidising.