Answer:
Sodium
Explanation:
Sodium is a pure chemical, hence it can be represented by the single chemical symbol Na.
Water has a chemical symbol of H2O, consisting of two, and not one, symbols, H and O.
Sulfate ion consists of a sulfide ion and four oxygen ions (SO4)2-. Hence, it does not fit.
Finally, potassium chloride has a chemical symbol of KCl. Consisting of two different chemical symbols, (K and Cl), it does not fit either.
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