The correct answer is d) magnesium phosphite.
To break it down, the chemical formula Mg3(PO3)2 indicates that there are three magnesium (Mg) ions and two phosphite (PO3) ions. In this compound, magnesium acts as a cation while phosphite is the anion. Knowing the naming conventions in chemistry, the cation name is stated first followed by the anion name. As the compound contains phosphite, it specifically refers to magnesium phosphite rather than magnesium phosphate or any other variant, making option d the correct choice.