An unknown substance dissolves in water to give a basic solution and gives a gas with acid that turns lime water milky. Which substance could it be?

Correct answer: Sodium carbonate

A gas that turns lime water milky is carbon dioxide, so the unknown substance should contain a carbonate or hydrogencarbonate ion. Sodium carbonate reacts with an acid to release CO2 and forms a basic aqueous solution because carbonate ions hydrolyse water to produce hydroxide ions. Sodium chloride is nearly neutral, ammonium chloride is acidic, and copper sulfate is not the expected carbonate source. Hence option A is correct.