Explanation

Of the elements listed, aluminum is the one that most commonly exhibits a +3 oxidation state.

Sodium typically shows a +1 oxidation state, and magnesium typically shows a +2 oxidation state.

While iron can exist in a +3 oxidation state (as in iron(III) chloride),

It also commonly exists in a +2 oxidation state (as in iron(II) sulfate), making it a transition metal with variable oxidation states.