If a salt solution turns red litmus blue, which could be a possible example?

Correct answer: Sodium carbonate

The governing concept is hydrolysis of salts and the resulting solution character. Sodium carbonate is formed from a strong base, sodium hydroxide, and a weak acid, carbonic acid. Its carbonate ions hydrolyse in water and generate hydroxide ions, so the solution is basic and can turn red litmus blue. Ammonium chloride and copper sulphate generally give acidic solutions, while sodium chloride is approximately neutral. Hence sodium carbonate, option A, is the suitable example.