Which of the following is a network solid?

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Allotropy is a phenomenon where an element can exist in two or more forms that have different physical properties and similar or almost similar chemical properties.
Carbon exhibits two allotropic forms:
Crystalline – eg diamond, graphite, fullerene.
Amorphous- coal, coke, lampblack, wood charcoal, sugar charcoal, bone charcoal, gas carbon, etc.

Complete answer:
Step 1
We have seen that diamond, graphite, and fullerene are the crystalline form of carbon. Diamond is very hard, transparent and non-conductor of electricity whereas graphite on the contrary is soft, opaque and a good conductor of electricity.
Step 2
Diamond has a three-dimensional network of carbon atoms joined tetrahedrally with each other through strong covalent bonds. Each carbon atom is in a state of \[s{p^3}\]hybridization and has a tetrahedral linkage to four neighboring carbon atoms. This network extends in all the three dimensions forming an open cage-like structure. The network formed by the atoms in-network solid is very hard as it forms strong covalent bonds. For this reason, the diamond is extremely hard and has a very high melting point.
Step 3
When the atoms in a chemical element or a compound are bonded by covalent bonds and form a continuous network extended throughout the material is known as a network solid or a covalent network solid. A network solid is considered as a macromolecule as there are no individual molecules. Example, silicon dioxide or quartz, diamond, graphite, mica, etc.

Hence, you can find that option c) is the correct answer for the given question.

Note:Graphite another crystalline allotrope of carbon is also an example for a network solid. Graphite is a macromolecule of carbon atoms covalently bonded to three other carbon atoms forming hexagonal sheets. Several sheets of graphite molecules are held together by a weak van der Waal force of attraction.