A hard substance melts at high temperature and is an insulator in both solid and in molten state. This solid is most likely to be a/an :

Solution

Chemistry ma’am · 3 steps

Hard and high-melting leaves ionic or covalent; the molten-state test decides between them.

1. Hard and high melting. A solid is hard and melts at a high temperature only when its particles are held by strong bonds through the whole crystal. That points to an ionic or a covalent network solid, not a molecular one.
2. Check conduction. To conduct, a solid needs charges that can move, either free electrons or mobile ions. Compare each type in the solid and in the molten state.

   Solid type	Hardness and melting point	Conducts as solid	Conducts when molten
   Covalent (diamond, SiO₂)	very hard, very high m.p.	no	no
   Molecular (ice, I₂)	soft, low m.p.	no	no
   Ionic (NaCl)	hard, high m.p.	no	yes, the ions become free to move
   Metallic (Cu)	hard to soft, usually high m.p.	yes	yes

3. So. Only a covalent network solid is hard, high melting, and an insulator both as a solid and as a melt, because its atoms are joined by covalent bonds and have no ions or free electrons to carry charge. The answer is (a).

The idea to keep. Use the molten state as the tie-breaker: ionic solids insulate as solids but conduct when melted, while covalent network solids insulate in both states.

Answer from the NTA final answer key. Solution by PrepWiser.