An ionic bond forms between a metal and a nonmetal. So, since C and O are nonmetals it cannot be that one. Also, P and Cl are also both nonmetals so it cannot be that one either. That leaves Li and O and Ca and Br. In an ionic bond metals loose electrons and nonmetals gain electrons. Therefore it cannot be Li and O because it states that oxygen, the nonmetal, looses electrons and lithium, the metal, looses electrons. So, the answer has to be Ca and Br because Ca, the metal, looses electrons and Br, the nonmetal, gains electrons.
