Question

Inert pair effect is predominant in:

Answer 1

The inert-pair effect is the tendency of the two electrons in the outermost atomic $s -$orbital to remain unshared in compounds of post-transition metals. The term inert pair effect is often used in relation to the increasing stability of oxidation states that are two less than the group valency for the heavier elements.

Complete answer:
The lead inert pair effect is most prominent because as we move down the group from carbon to lead the tendency of non-participation inner ns electrons in chemical bonding increases due to the large size of atoms. So, $Pb$ is having more tendency of showing an inert pair effect.
The pair of electrons in the valence $s -$orbital is reluctant to take part in bond formation due to poor shielding effect of $d -$electrons and $f -$electrons in heavier elements. It is called inert pair effect due to which lower oxidation state becomes more stable than higher oxidation state in case of $p -$block elements. The inert pair of $ns$ electrons remains more tightly held by the nucleus and hence participates less in bond formation.
The inert pair effect is due to the decrease in bond energies down a group. The $ns$ electrons do not become significantly more inert lower in energy as one descends the groups of the periodic table.

Note:
The inert pair effect is not valid to transition elements because this effect only depends upon the $s -$orbital electrons and they do not have them. Moreover, the $d -$orbital electrons in transition elements are degenerate I.e. have the same energy, so, the inert pair effect cannot be valid to them.