Which of the following is most easily oxidized?
A. Ni2+
B. Fe3+
C. H+
D. Fe2+

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.