Properties identify X as Na, forming NaCl with Y=Cl Steps:

• Period 3 elements with solid giant ionic oxide: Na, Mg, Al.
• Eliminate Al: Al2O3 giant but AlCl3 hydrolyzes with water (AlCl3 + 3H2O → Al(OH)3 + 3HCl).
• Eliminate Mg: MgO giant but MgCl2 shows partial hydrolysis in water.
• X=Na fits all (Na2O giant ionic solid, NaCl inert to water); Y=Cl (period 3), compound=NaCl. Why C is correct:
• NaCl is the 1:1 ionic compound from Na (X) and Cl (Y), matching the non-reactive chloride property per Fajans' rules for group 1. Why the others are wrong:
• A. AlCl3 reacts with water via hydrolysis, violating the condition.
• B. MgS uses Mg, whose chloride partially hydrolyzes (Mg²⁺ + H2O ⇌ MgOH⁺ + H⁺).
• D. PCl3 hydrolyzes (PCl3 + 3H2O → H3PO3 + 3HCl); P4O10 molecular, not giant.

Final answer: C