Concept:Hardness of water is caused by the presence of certain dissolved metal ions.Explanation:The ions responsible for making water hard are mainly calcium ions (Ca2+) and magnesium ions (Mg2+).These ions come from dissolved minerals such as limestone and dolomite.Hard water does not contain high amounts of nitrogen, phosphorus, or helium ions.Therefore, the missing term is magnesium ions.Answer:A. magnesium ions