Most likely to form an ionic compound?

Barium is a metal in the second group so to form an ionic compound, you’ll need to find an element with a corresponding number of free electrons, i.e. 2. Metals cannot form ionic compounds with metals, so K and Zn are out. C is more inclined towards covalence bonds. That leaves us with F, which has a very high electronegativity and can form BaF2.