Element X is magnesium in period 3, forming a basic oxide solution.

Steps:

• Three occupied shells place X in period 3 (Na to Ar).
• Atomic radii decrease across period 3; Na has the largest, so second largest is Mg (group 2).
• Mg oxide is MgO, which reacts with water: MgO + H₂O → Mg(OH)₂.
• Mg(OH)₂ is a weak base, producing OH⁻ ions for pH > 7.

Why C is correct:

• Basic oxides of group 2 metals like MgO form weakly alkaline solutions with pH around 9, per the Arrhenius definition of bases increasing [OH⁻].

Why the others are wrong:

• A: pH 5 indicates acidity, but metal oxides produce basic solutions.
• B: pH 7 is neutral, not matching the basic nature of group 2 oxides.
• D: pH 14 is strongly basic, exceeding the weak basicity of Mg(OH)₂.

Final answer: C