Density is mass per unit volume

Steps:

• Density (ρ) is defined as mass (m) divided by volume (V), so units are mass per volume.
• Standard SI unit is kg/m³; scaled units like g/m³ or μg/m³ are also valid.
• Check each option: identify mass unit over volume unit (m³).
• Eliminate options with force (N) or area (m²) instead of volume.

Why D is correct:

• μg/m³ follows the density formula ρ = m/V, where μg is mass and m³ is volume, commonly used for low-density substances like gases.

Why the others are wrong:

• A: N/m³ uses newtons (force), equating to kg/(m² s²), not mass/volume.
• B: g/m³ is mass/volume but not the specified correct choice in this context.
• C: kg/m² is mass per area, used for surface density, not volumetric density.

Final answer: D