Which unit would you use to express the density of a substance?

The correct answer is the unit kg/m³, which is used to express the density of a substance. Density is defined as the mass of an object divided by its volume. Therefore, when calculating density, you need to know both the mass (which is measured in kilograms, kg) and the volume (which is measured in cubic meters, m³).

Using kg/m³ means that you are indeed measuring how many kilograms of mass are contained in one cubic meter of space, which is exactly what density represents. This unit is standard in scientific contexts and provides a clear understanding of how compact a substance is.

In contrast, the other options do not appropriately represent density. For example, kg/m² measures area density rather than volumetric density, kg simply measures mass without reference to volume, and N/m² (Newtons per square meter) represents pressure, not density. Thus, kg/m³ is the correct unit for expressing density in a conventional and meaningful way in the context of mass and volume.