Answer:
B: Fe3+
Explanation:
Usually in oxidation reactions, the oxidation number of a monatomic ion is the same as the charge on the ion, for example: oxidation number for Na^(+) is +1 , while in S^(2-), oxidation number is -2.
Now, from the activity series of metals in aqueous solution which I attached, we will see that iron (Fe) is easily oxidized than Nickel (Ni2+) and hydrogen (H+).
So we are left with Fe^(2+) and Fe^(3+).
Now, an increase in oxidation number means the more likely to be oxidized.
Thus, Fe^(3+) is the most easily oxidized.