Element X is sodium, a Period 3 alkali metal with metallic bonding

Steps:

• Period 3 elements include Na to Ar, spanning metals, metalloids, and non-metals.
• Rapid reaction with water forming alkaline solution (e.g., Na + H₂O → NaOH + H₂) identifies Group 1 alkali metal.
• In Period 3, this is sodium (Na).
• Sodium's metallic structure allows evaluation of options based on metal properties.

Why A is correct:

• Metals conduct electricity when molten as delocalized electrons move freely, carrying charge (metallic bonding definition).

Why the others are wrong:

• B: Low melting points under 100°C typically indicate simple molecular covalent structures, not metallic bonding in sodium.
• C: Sodium atoms form metallic bonds with delocalized electrons, not localized covalent bonds.
• D: The given reaction is with water (temperature unspecified); sodium reacts vigorously but option adds unconfirmed "cold" detail.

Final answer: A