Add electron dots and charges as necessary to show the reaction of calcium and oxygen to form an ionic compound.

Nuetral Atoms--> Ions

Ca+0-->Ca+0

So the problem ask to show the reaction of calcium and oxygen to form an ionic compound and the best answer base on the element you give is the Ca loses 2 electrons to achieve a noble gas electron configuration and the oxygen gains 2 electrons also. I hope you are satisfied with my answer and feel free to ask for more