Ammonium ion's acidity dissolves Mg(OH)₂ via proton donation

Steps:

• Mg(OH)₂ is a sparingly soluble base that provides OH⁻ ions.
• In NH₄Cl solution, NH₄⁺ acts as a weak acid, donating H⁺ to OH⁻ to form NH₃ and H₂O.
• This reaction shifts Mg(OH)₂ dissolution equilibrium rightward, increasing solubility.
• NaCl lacks an acidic cation, so no such reaction occurs, and Mg(OH)₂ remains insoluble.

Why D is correct:

• NH₄⁺ is a Brønsted-Lowry acid (NH₄⁺ ⇌ NH₃ + H⁺), protonating OH⁻ to remove it from solution per Le Châtelier's principle.

Why the others are wrong:

• A: Ionic radius affects lattice energy but not the acid-base dissolution mechanism here.
• B: Both NaCl and NH₄Cl fully dissociate in water; dissociation extent is irrelevant.
• C: Equal electrons (10 each) implies similar size but doesn't explain selective solubility.

Final answer: D