An oxidizing agent is a substance that causes another substance to lose electrons. The stronger the oxidizing agent, the more easily it can oxidize other substances.

The oxidizing ability of a substance depends on its electrode potential. The electrode potential of a substance is a measure of its tendency to undergo reduction. The higher the electrode potential, the stronger the oxidizing agent.

The electrode potentials of the given substances are as follows:

FeO4-2: +2.20 V

Fe2O3: +1.24 V

Fe3O4: +0.87 V

FeO: -0.87 V

As you can see, FeO4-2 has the highest electrode potential, which means that it is the strongest oxidizing agent of the given set.