Ionic compounds typically compose of a metal and a non metal. So you can easily cancel out B and D. You are now left with two more choices.
When it comes to Ionic compounds just remember that
the LESS valence electrons an element has, the higher the tendency to LOSE them;
the MORE valence electrons an element has, the higher the tendency to RECEIVE them.
Metals have less valence electrons, so they tend to lose electrons, when there is a loss of electron, the atom becomes a CATION. Non-Metals on the other hand, have more valence electrons, so they tend to receive electrons, when there is a gain of electrons, the atom becomes an ANION.
So your answer is C.
