The solubility of a substance is 60 g per 100 mL of water at 15°C. A solution of this substance is prepared by dissolving 75 g in 100 mL of water at 75°C. The solution is then cooled slowly to 15°C without any solid separating. The solution is (a) supersaturated at 75°C (b) supersaturated at 15°C (c) unsaturated at 15°C (d) saturated at 15°C.