among Pb^(2+), Ge^(2+), Sn^(2+) and Si^(2+) whuch is most stable
Inert pair effect may be defined as inertness of inner ns sub-shell towards chemical reaction. It is seen in groups 3A and 4A especially (5s and 6s electrons). Therefore shows mainly +2 oxidation state frequently as compared to +4oxidation state. So as we move down the group, due to inert pair effect lower oxidation state become more stable.