Answer: C. The substance must be alkaline — adding it must increase the \(\text{OH}^{-}\) concentration (or decrease \(\text{H}^{+}\)) compared to neutral water.
Sodium oxide is a basic oxide: \(\text{Na}_2\text{O} + \text{H}_2\text{O} \rightarrow 2\text{NaOH}\). The resulting solution is strongly alkaline, with \([\text{OH}^{-}] > [\text{H}^{+}]\). Answer C.
Ammonium chloride (A) is a slightly acidic salt; carbon (B) doesn't react with water; sulfur dioxide (D) is an acidic oxide forming \(\text{H}_2\text{SO}_3\). None of these raise the pH above 7.
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