Which characteristic describes giant covalent structures?

Giant covalent structures are characterized by a large network of atoms that are all interconnected by strong covalent bonds. This extensive bonding network results in a solid structure with high stability and strength. The strength of these covalent bonds means that a substantial amount of energy is required to break them, which is why giant covalent structures often have high melting and boiling points.

In contrast, the other characteristics do not apply to giant covalent structures. For instance, having low melting points typically describes molecular substances with weaker intermolecular forces, not the strong covalent bonds found in giant covalent networks. Additionally, the presence of ions connected by ionic bonds is indicative of ionic compounds, rather than covalent structures. Lastly, free-flowing electrons are a feature of metallic bonding or covalent compounds with delocalized electrons, but not of giant covalent structures where all atoms are held by discrete, strong covalent bonds.