Final answer:
Diamond is an example of a covalent-network solid, characterized by a hard structure and very high melting point, making it the correct option out of the given choices.
Step-by-step explanation:
Covalent bonding occurs in various types of solids, but when we talk specifically about covalent-network solids, we are referring to a class of materials where atoms are interconnected in a large, continuous network entirely by covalent bonds. Examples of these materials include diamond, silicon carbide, and silicon dioxide.
These substances stand out because of their hardness, high melting points, and strength. Among the options provided, such as Sodium chloride (A), Diamond (B), Iron (C), and Water (D), the correct option that represents a covalent-network solid is Diamond (B). Diamond is comprised entirely of carbon atoms, each of which is tetrahedrally bonded to four other carbon atoms, creating an extensive lattice that is exceptionally tough and resistant to heat.