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

Question

$\begin{array}{1 1} (1)Sn^{2+} \;is reducing\; while\; Pb^{4+} \; is\; oxidising \\ (2)Sn^{2+} \;is\; oxidising\; while \;Pb^{4+}\; is\; reducing\\ (3) Sn^{2+}\; and\; Pb^{2+}\; are\; both\; oxidising\; and\; reducing\; Sn^{4+}\; is\; reducing\; while\; Pb^{a+}\; is\; oxidising \end{array} $