Ionic compounds contain ions and are held together by the attractive forces among the oppositely charged ions. It's common to see ionic compounds formed by metal and nonmetal because you can note that the electronegativity difference between the elements of these compounds is greater than or equal to 1.7.
In the group of answer choices, you can see various things about Mg - Cl because it's the only answer that contains a metal (Mg) and nonmetal (Cl), their difference electronegativity is 1.8 (The electronegativity of Cl is 3.0 and for Mg is 1.2, so 3.0 - 1.2 = 1.8, it's greater than 1.7. The electronegativities can be found in the periodic table). Mg has a positive charge (+2) and Cl has a negative charge (-1).
All these characteristics is telling us that the bond Mg - Cl is the more ionic in this case. The answer is Mg - Cl.