Which set of atoms will form an ionic compound? A) one calcium atom with an oxygen atom B) two hydrogen atoms with one oxygen atom C) one oxygen atom with another oxygen atom D) two phosphorous atoms with five oxygen atoms

The correct answer is A. The Ionic compound is held by ionic bonding and this ionic bond forms between a metal and a non-metal. Here the calcium atom is the metal and the oxygen atom is a non-metal this set of atoms will form an ionic compound.