Final answer:
Graphite has the highest melting point among the given substances due to its covalent network solid structure, with extremely strong carbon-carbon bonds.
Step-by-step explanation:
The substance with the highest melting point among graphite (A), sodium chloride (B), water (C), and mercury (D) is graphite. The melting points of the substances in question can be inferred from the patterns observed in the chemistry of elements and compounds. As graphite is a form of carbon in a covalent network solid structure, it has extremely strong carbon-carbon bonds, making it very stable and giving it a high melting point. Graphite's melting point is around 3652 - 3697 °C, which is significantly higher than sodium chloride (NaCl, melts at 801 °C), water (H2O, melts at 0 °C), and mercury (Hg, melts at -38.83 °C).
To understand the high melting points of such substances, it is important to consider their intermolecular forces. Sodium chloride features ionic bonds, which are strong but not as strong as the covalent bonds in graphite. Water has hydrogen bonds, which, while the strongest of the intermolecular forces, are weaker than ionic bonds. Lastly, mercury as a metal has a relatively weak metallic bond in its liquid state at room temperature. Hence, among the given substances, graphite definitely stands out with the highest melting point due to its strong covalent network.