The correct answer is kilogram per cubic metre.

Density is defined as the mass per unit volume of a substance. The formula for density is:

\(Density = \frac{Mass}{Volume}\)

The SI unit of mass is the kilogram (kg), and the SI unit of volume is the cubic metre (m³). Therefore, the SI unit of density is kilograms per cubic metre (kg/m³).

Let's examine why the other options are incorrect:

• Metre cubed (m³) is the unit of volume, not density.
• Metre squared (m²) is the unit of area, not density.
• Kilogram metre per second (kg⋅m/s) is the unit of momentum, not density.

Understanding the fundamental definition of density and the units used to measure mass and volume is crucial for determining the correct unit of density. This concept is foundational in physics and chemistry, with applications in diverse fields like material science and fluid mechanics. The correct unit reflects the intrinsic property of a substance: how much mass is packed into a given volume. Remember that the density of a material can vary with temperature and pressure.