First you need to know how ionic compounds are created!
Ionic compounds are formed by IONS (Obviously, what am I, a dummy?)
There are two types of ions: anions and cations.
Anions can only be formed by nonmetals, and cations can only be formed by metals.
In order for ions to bond, there must be one anion and one cation, because opposite charges attract.
Let's look at your choices.
1) Ca + Br can form ionic bonds
2) Na + F can form ionic bonds
3) Li + Cl can form ionic bonds
However...
4) Mg + Ar
This is a special case. Ar is a NOBLE GAS, which means that it has the maximum number of valence electrons (8) in its outermost shell. Therefore, ANY type of bonding is IMPOSSIBLE.
Your answer should be 4) Mg + Ar
