Substance X exhibits metallic bonding properties

Steps:

• High melting (127°C) and boiling (295°C) points indicate strong intermolecular forces typical of giant structures.
• Malleability suggests layers of atoms that can slide without breaking bonds, characteristic of metals.
• Good electrical and thermal conductivity at room temperature implies delocalized electrons, a feature of metallic lattices.
• These properties collectively match a giant metallic structure.

Why C is correct:

• Giant metallic lattices consist of positive ions in a sea of delocalized electrons, enabling high conductivity via electron mobility and malleability via ion sliding (metallic bonding definition).

Why the others are wrong:

• A. Giant covalent: Typically brittle and poor conductors due to localized electrons, unlike X's malleability and conductivity.
• B. Giant ionic: High melting/boiling points but brittle solids that don't conduct electricity at room temperature.
• D. Simple molecular: Low melting/boiling points from weak van der Waals forces, and insulators.

Final answer: C