Final answer:
The units of density can be expressed in different forms such as kilograms per cubic meter (kg/m³) or grams per cubic centimeter (g/cm³). The unit mg/cm² is not a valid unit of density.
Step-by-step explanation:
In physics, the unit of density can be expressed in different forms. The SI unit of density is kilograms per cubic meter (kg/m³). However, in everyday usage, densities of solids and liquids are often expressed in grams per cubic centimeter (g/cm³) or grams per milliliter (g/mL). On the other hand, densities of gases are typically reported in grams per liter (g/L).
Based on this information, option B) mg/cm² would not be a unit of density because it refers to milligrams per square centimeter, which does not have the dimensions of mass over volume like the other options.