Neutral pH requires soluble salts without acidic ammonium ions or insoluble products Steps:

• Identify NPK sources: Nitrogen from nitrates/ammonium, phosphorus from phosphates, potassium from nitrates/sulfates.
• Ensure water solubility: All components must dissolve fully without precipitates.
• Check pH 7: Avoid acidic (ammonium/sulfate) or strongly basic/unbalanced salts; neutral nitrates and buffered phosphates work.
• Evaluate options for N, P, K coverage, solubility, and neutrality.

Why D is correct:

• Potassium nitrate (KNO3) provides neutral N and K; sodium phosphate (as buffered Na2HPO4/NaH2PO4) provides P at pH 7 per Henderson-Hasselbalch equation for phosphate buffer.

Why the others are wrong:

• A: Phosphorus oxide (P4O10) reacts with water to form acidic H3PO4 (pH <7) and is not directly soluble.
• B: Ammonium phosphate hydrolyzes to release NH4+ and H+, yielding acidic solution (pH ~4-5).
• C: Ammonium sulfate is acidic (NH4+ hydrolysis); Ca2+ and PO43- form insoluble Ca3(PO4)2 precipitate.

Final answer: D