The correct answer is d) Magnesium.
To understand why magnesium has an oxidation number of +2, we need to look at its position on the periodic table. Magnesium is in group 2, also known as the alkaline earth metals. Elements in this group typically lose two electrons when they form compounds, resulting in a charge of +2.
In contrast, helium is a noble gas and does not typically form compounds, so it doesn’t have an oxidation number like +2. Sodium and potassium, which are in group 1 of the periodic table, typically have an oxidation number of +1 when they form compounds, as they each lose one electron. Therefore, magnesium is the only element in this list that commonly exhibits an oxidation state of +2.