Density is defined as which of the following?

• A. D = M / V
• B. D = V / M
• C. D = M + V
• D. D = M - V

Answer: (A)

Density measures how much matter is packed into a given amount of space. It is defined as mass per unit volume, so density equals mass divided by volume. This means that if you have more mass in the same volume, the density goes up; if you keep the mass the same but increase the volume, the density goes down. The standard relation is D = M / V, with units like kg/m^3 in SI (or g/cm^3 for common solid substances). The other ideas—adding or subtracting mass and volume, or taking volume over mass—don’t describe how much matter sits in a space, and taking the reciprocal (volume over mass) would give a different quantity entirely.