(a) X = Na₂CO₃, Y = NH₄Cl
Phenolphthalein turns pink in basic solutions. Na₂CO₃ is a salt of strong base and weak acid, so its solution is basic (turns pink). NH₄Cl is a salt of strong acid and weak base, so its solution is acidic/neutral (no colour change).
Source: Chapter 2, Section 2.1.1
Phenolphthalein is colourless in acidic/neutral solutions and turns pink in basic solutions. Na₂CO₃ undergoes hydrolysis to give a basic solution; NH₄Cl gives an acidic solution. Options (b), (c), (d) are wrong because Na₂SO₄ and NaNO₃ give neutral solutions, and NaHCO₃ also gives a mildly basic solution — but the only pair where X is clearly basic (turns pink) and Y is acidic/neutral (no change) is option (a). Examiners expect you to recall the nature of salt solutions based on the strength of parent acid and base.