The O4 has an oxidation state of -8 and since the molecule of iron oxide is electrically neutral ,the sum of the O.S of the three iron atoms must be +8. The only way to achieve this is by having 2Fe atoms with an O.S of +3 and 1Fe atom with an O.S of +2 per molecule.
Therefore, 1/3 of the iron ions are Fe+2.
Answer is thus B.
Hope it helps:)
thanks so much @Ujjwal !