Ionic bonds typically form between metals and nonmetals, where one atom donates an electron to another atom.
In the options provided:
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K and Cl: Potassium (K) is a metal and chlorine (Cl) is a nonmetal, so they are likely to form an ionic bond.
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C and O: Carbon (C) is a nonmetal and can form covalent bonds with oxygen (O), not ionic bonds.
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S and Cl: Both sulfur (S) and chlorine (Cl) are nonmetals, so they are more likely to form covalent bonds rather than ionic bonds.
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K and Ca: Both potassium (K) and calcium (Ca) are metals, so they do not form ionic bonds with each other.
Therefore, the correct response is K and Cl, as they are the most likely to form an ionic bond.