(D) B < A < C
Ammonium chloride (strong acid + weak base → acidic, pH < 7), potassium nitrate (strong acid + strong base → neutral, pH = 7), sodium carbonate (strong base + weak acid → basic, pH > 7).
From section 2.4.2: salts of strong acid + weak base are acidic (pH < 7); strong acid + strong base are neutral (pH = 7); strong base + weak acid are basic (pH > 7). NH₄Cl = HCl + NH₄OH (weak base) → acidic; KNO₃ = HNO₃ + KOH (both strong) → neutral; Na₂CO₃ = NaOH + H₂CO₃ (weak acid) → basic. So ascending pH order: B < A < C.