Answer: A (sulfur dioxide and carbon dioxide only). The solution is alkaline (pH 10), so to decrease the pH we must add something acidic that neutralises some of the sodium hydroxide.
Sulfur dioxide is an acidic gas: \(\text{SO}_2 + \text{H}_2\text{O} \to \text{H}_2\text{SO}_3\) (sulfurous acid), which neutralises the alkali and lowers the pH. Carbon dioxide is also acidic: \(\text{CO}_2 + 2\text{NaOH} \to \text{Na}_2\text{CO}_3 + \text{H}_2\text{O}\), again lowering the pH. So statements 1 and 2 both decrease the pH.
Calcium oxide is a basic oxide: \(\text{CaO} + \text{H}_2\text{O} \to \text{Ca(OH)}_2\), which is alkaline and would raise the pH, not lower it, so statement 3 is wrong. Only 1 and 2 decrease the pH, which is option A. The common error is assuming any metal oxide is acidic, but CaO is basic.
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