The correct option is D All of the above.
Covalent Solids:
A wide variety of crystalline solids of non-metals result from the formation of covalent bonds between adjacent atoms throughout the crystal.
They are also called giant molecules. Covalent bonds are strong and directional in nature, therefore atoms are held very strongly at their positions. Such solids are very hard and brittle. They have extremely high melting points and may even decompose before melting. They are insulators and do not conduct electricity (except graphite).
Diamond ,graphite,silicates are typical examples of such solids.