Final answer:
Density of flat objects like a metal sheet is usually measured in mass per area, like kilograms per square meter, though conventional density is mass per volume, such as grams per cubic centimeter for solids and liquids.
Step-by-step explanation:
The density of flat objects such as a square sheet of metal is measured in units of mass per area. The most appropriate units listed in your question for expressing the density of the sheet would be kilograms per square meter (kg/m2), although this is not a conventional unit for density. Regular density is defined as mass per unit volume, and in the International System of Units (SI), it is expressed as kilograms per cubic meter (kg/m3). However, for solids and liquids, it's more common to use grams per cubic centimeter (g/cm3). Since your question involves a flat or two-dimensional object, you would not typically use volume-based density but rather mass per unit area if you're measuring the 'density' of a sheet or layer of material.