Covalent bonds are typically formed between nonmetals, which share electrons to achieve stability, rather than metals which tend to lose electrons and form ionic bonds.
Given the options and their positions in the periodic table:
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Sodium (Na) and Bromine (Br) - Sodium is a metal and bromine is a nonmetal (ionic bond).
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Magnesium (Mg) and Sulfur (S) - Magnesium is a metal and sulfur is a nonmetal (ionic bond).
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Nitrogen (N) and Oxygen (O) - Both nitrogen and oxygen are nonmetals, making it likely they will form covalent bonds with each other.
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Potassium (K) and Iodine (I) - Potassium is a metal and iodine is a nonmetal (ionic bond).
Based on this analysis, nitrogen (N) and oxygen (O) are most likely to form covalent bonds with each other.