Substances that contain a transition metal can be oxidizing. The greater the number of oxidation of this metal, the greater the oxidizing character of the substance. In an industrial process in which the use of an oxidizing agent is necessary, only five substances are available: CaO, FeO, Cu₂O, Cr₂O₃ and KMnO₄. The substance that should be used in this process, as it has a greater oxidizing character, is:

a) Cr₂O₃

b) Cu₂O

c) CaO

d) FeO

e) KMnO₄