It is because of inability of $ns^2$ electrons of the valence shell to participate in bonding that :

Question

( A ) $Sn^{2+}$ is oxidising while $Pb^{4+}$ is reducing
( B ) $Sn^{2+}$ is reducing while $Pb^{4+}$ is oxidising
( C ) $Sn^{2+}$ and $Pb^{2+}$ are both oxidising and reducing
( D ) $Sn^{4+}$ is reducing while $Pb^{4+}$ is oxidising