The reducing power of divalent species decreases in the order:

The inert pair effect is the unwillingness of the s-electrons to participate in bonding. This effect is majorly seen in heavier metals where s-electrons show inertness and do not involve in bonding with the other elements.

Pb, Ge, and Sn belong to Group-14 and show two oxidation states: +2 and +4. Due to the inert pair effect, the stability of the +2 oxidation state increases and the +4 oxidation state decreases. Pb is heavier than Ge and Sn, thereby effectively showing the inert pair effect. This is the reason that Pb is more stable in the +2 than the +4 state. The trend of stability of +2 oxidation state of Pb, Ge, and Sn is:
${\mathrm{Pb}}^{2+}>{\mathrm{Sn}}^{2+}>{\mathrm{Ce}}^{2+}$
Hence, the reducing power trend of Ge, Pb, and Sn is:

 $\mathrm{Ge}>\mathrm{Sn}>\mathrm{Pb}$