The incorrect option out of the following is :

The incorrect option out of the following is ${\mathrm{IP}}_1\text{ of }\mathrm{Pb}<{\mathrm{IP}}_1\text{ of }\mathrm{Sn}$.

The $Z_{eff}$ for the 6p and 5p electrons in lead and tin are both equal, according to Slater's rule. But there is a difference in the ionization energy.

Hence, option C is correct.