Assuming complete ionization, same moles of which of the following compounds will require the least amount of acidified KMnO4 for complete oxidation?

Assuming complete ionization, the same moles of FeSO4 will require the least amount of acidified KMnO4 for complete oxidation. This is because the oxidation state of iron in FeSO4 is +2, and it only needs to be oxidized to +3. In contrast, the other compounds have iron in lower oxidation states and/or other oxidizable components that require more electrons to be transferred during oxidation. Let's consider the balanced redox reactions for each compound:

FeC2O4 involves the oxidation of both Fe(II) and C2O4(2-), requiring more KMnO4.
FeSO3 involves the oxidation of Fe(II) and SO3(2-), requiring more KMnO4.
Fe(NO2)2 involves the oxidation of Fe(II) and NO2(-), requiring more KMnO4.
FeSO4 only involves the oxidation of Fe(II) to Fe(III), requiring the least KMnO4.