Final answer:
Substance A is a metallic solid, Substance B is an ionic solid, Substance C is a covalent network solid, and Substance D is a molecular solid, categorized by their respective properties such as electrical conductivity, malleability, hardness, and melting points.
Step-by-step explanation:
Identifying the type of crystalline solid for each substance based on their given properties is a common task in chemistry. For Substance A, which is malleable, ductile, conducts electricity, and has a high melting point, it is characteristic of metallic solids.
These properties come from the sea of delocalized electrons that allow metals to conduct electricity and to be shaped without breaking.
Substance B, being brittle, not conducting electricity as a solid but doing so when molten, and having a very high melting point, fits the description of ionic solids. Ionic solids form a 3D lattice of positively and negatively charged ions, which can only move freely to conduct electricity when molten.
Substance C is hard, does not conduct electricity, and has a very high melting point, pointing towards a covalent network solid. These solids have a network of covalent bonds that create strong and hard materials, such as diamonds.
Lastly, Substance D is soft, non-conductive, and has a relatively low melting point, indicative of a molecular solid. Molecular solids are made up of molecules held together by intermolecular forces, which are weaker than covalent or ionic bonds.