The stability of the lower oxidation state increases down a group...

The elements given belong to Group 14 of the periodic table: C, Si, Sn, Pb.

The general oxidation states exhibited by Group 14 elements are +2 and +4. Down the group, the stability of the lower oxidation state (+2) increases and that of the higher oxidation state (+4) decreases due to the inert pair effect.

For lead (Pb), the +2 oxidation state is much more stable than the +4 oxidation state. In $PbO_{2}$, lead is in the +4 oxidation state, which is highly unstable. Therefore, $PbO_{2}$ readily accepts electrons to get reduced to the more stable $Pb^{2+}$ state ($PbO$).

Since it easily undergoes reduction, $PbO_{2}$ acts as a very powerful oxidizing agent compared to $CO_{2}$, $SiO_{2}$, and $SnO_{2}$, where the +4 oxidation state is relatively stable.

Answer: $PbO_{2}$