Base-neutralizing substances lower pH of NaOH solution

Steps:

• Sodium reacts with water: 2Na + 2H₂O → 2NaOH + H₂, forming basic solution Y (pH > 7).
• Adding X lowers pH, so X must provide H⁺ or consume OH⁻.
• MgCl₂ → Mg²⁺ + 2Cl⁻; Mg²⁺ hydrolyzes (Mg(H₂O)₆²⁺ ⇌ Mg(H₂O)₅OH⁺ + H⁺), generating acidity.
• Al(OH)₃ + OH⁻ → [Al(OH)₄]⁻, dissolving in base and consuming OH⁻ to reduce pH.

Why A is correct:

• MgCl₂ and Al(OH)₃ (assuming A lists these) both acidify or neutralize the basic Y via hydrolysis or amphoteric reaction.

Why the others are wrong:

• B: MgCl and OH are invalid/incomplete; no pH effect possible.
• C: NaCl neutral (no ions affect pH); Al OH incomplete.
• D: NaCl neutral; KO nonexistent compound.

Final answer: A