Which of the following would you expect to form an ionic bond when combined?

In basic terms an ionic bond forms between a metal and a non-metal.
So with your list the only mixture with a metal and non-metal is Mg and Cl, which would form magnesium chloride MgCl2.
Hope this helps.

You need to pick the most electronegative and electropositive elements.  If you remember from periodic trending, Francium (Fr) is the most electropositive metal and fluorine (F) is the most electronegative non-metal.  The main criteria to determine if a compound is ionic is one element needs to be metal and another need to be nonmetal.  

Li and Na,  both are metals so this clearly can't be the answer, one needs to be an electronegative non-metal but both are electropositive.  Cannot form a salt.

Mg and Cl, Mg is 2nd row alkaline earth metal and fairly electropositive and Cl is very highly electronegative.  This can definitely form a salt (aka ionic bond).

P and O, these two elements are pretty close together in the periodic table and both non-metal.  Cannot be ionic.

H and Cl,  this is kinda a special case since hydrogen isn't a metal but its still pretty electropositive.  HCl is a known strong acid and all strong acids fully dissociate (ionize).  However, since H is a nonmetal, it is not technically form ionic bonds.

Only MgCl2 is ionic

Mg and Cl, ionic bonds only form between metals and nonmetals.

covalent bonds are between 2 nonmetals