Some substances allow electricity to pass in both solid and liquid forms due to free-moving electrons. Metallic substances are best known for this. Metals have a unique structure where electrons can move easily between atoms, even in the solid state. This makes them excellent conductors. When melted, metals still conduct electricity because their atoms stay surrounded by mobile electrons. In contrast, ionic substances conduct only when molten or dissolved, molecular solids are poor conductors, and covalent network solids are usually insulators. Therefore, the correct answer is metallic substances.