Period 3 oxides react with water to form basic or acidic solutions; mixture yields slightly acidic pH.

Steps:

• Period 3 oxides trend from basic (left) to acidic (right): Na₂O basic, P₂O₅ acidic, MgO basic, Al₂O₃ amphoteric/insoluble, SO₃ and P₄O₁₀ acidic.
• Basic oxides like Na₂O + H₂O → NaOH (pH >7); acidic oxides like P₂O₅ + H₂O → H₃PO₄ (pH <7).
• Mixture needs one basic and one acidic oxide to neutralize toward pH ~7, but "just below 7" implies slight excess acid.
• Option C fits: Na₂O (basic) + P₂O₅ (acidic) can balance to mildly acidic solution.

Why C is correct:

• Na₂O forms NaOH (base), P₂O₅ forms phosphoric acid (weak acid); equal moles yield pH ≈6.5–6.8, just below 7.

Why the others are wrong:

• A: Al₂O₃ insoluble (neutral), MgO weakly basic → pH neutral or slightly basic, not below 7.
• B: Na₂O and MgO both basic → strongly basic solution, pH >>7.
• D: SO₃ (H₂SO₄, strong acid) and P₄O₁₀ (H₃PO₄, acid) both acidic → very low pH, well below 7.

Final answer: C