Network solids are chemical compounds that are bound together by a covalent bond. Thus, option D is accurate.
Covalent bonds are the type of chemical bonds that mutually shares electrons between the atoms of the molecules of the compound.
A covalent crystal of network solids is held together by the covalent bond that can be seen in the structures of diamonds, graphite, silicon dioxide, etc.
Therefore, the network solids are option D. held by the covalent bonds.
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