The elements that are most likely to form an ionic bond are Na and Cl (sodium and chlorine).
Ionic bonds typically form between metals and nonmetals, where one atom donates electrons (the metal) and the other atom accepts those electrons (the nonmetal). Sodium (Na) is a metal and chlorine (Cl) is a nonmetal, making them suitable for ionic bonding.
The other pairs mentioned (C and O, Se and Cl, K and Ca) are not likely to form ionic bonds. Carbon (C) and oxygen (O) form covalent bonds, selenium (Se) and chlorine (Cl) can also form covalent bonds, and potassium (K) and calcium (Ca) are both metals, which typically do not form ionic bonds with each other.