Which of the following ions is in the lowest oxidation state?

Answer:
C. Fe in Fe₂O₃
Explanation:
The sum of the oxidations states times the atomicity is equal to the charge of the molecule.
A.
2.H + 1.P + 4.O = -1
2.(1+) + 1.P + 4.(2-) = -1
P = +5
B.
2.Cr + 7.O = -2
2.Cr + 7.(-2) = -2
2.Cr = 12
Cr = +6
C.
2.Fe + 3.O = 0
2.Fe + 3.(-2) = 0
2.Fe = 6
Fe = +3
Fe has the lowest oxidation state (+3).