Which of the following pairs of element could react to form an ionice compund

Ca and Cl
S and K
Ba and Cu
Cl and Br

Only CaCl, and KS form ionic compounds.

Ba and Cu both have metallic properties
same with Cl and Br, they both have non-metallic prperties.

Look at a periodic table with the valence electrons on it and find each element and the valence electrons 1-4 will recieve a positive charge and will lose electrons and 5-8 will recieve a negative charge and will gain electrons. If out of those two you have a postive element and a negative element then they will become an ionic compound

the answer is calcium and chlorine