Final answer:
Solid magnesium (Mg (s)) is classified as a metallic solid, which has a closely packed crystal structure and exhibits high electrical conductivity.
Step-by-step explanation:
When classifying the solid magnesium (Mg (s)), it would be considered a metallic solid. Magnesium is a metal and in solid form, it exists as a metallic solid with closely packed magnesium atoms that form a metallic crystal structure. This structure allows for the free flow of electrons, which imparts metallic solids with characteristic properties such as high electrical conductivity and malleability.