Identify the type of cyrstalline solid based on the following properties :
(a) Conducts electricity in the molten state but in the solid state.
(b) Very hard with melting point and acts as electrical insulator.
(c) Conducts electricity in the solid state, malleable and ductile.

Explanation

To identify the type of crystalline solid based on the given properties, we will analyze each property one by one.

### Step-by-Step Solution:

1. **Property (a): Conducts electricity in the molten state but not in the solid state.**
- **Analysis**: This property indicates that the solid does not conduct electricity when it is in its solid form, but it can conduct electricity when melted. This is characteristic of ionic solids. In ionic solids, the ions are fixed in place within the crystal lattice and cannot move freely, which is why they do not conduct electricity in the solid state. However, when melted, the ions are free to move, allowing the substance to conduct electricity.
- **Conclusion**: This type of solid is an **ionic solid**.

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