Answer: Density is mass per unit volume .
Explanation: Density is defined as the amount of mass in the space occupied (or volume)for some substance/material.
And you can calculate the density of some material by the equation:
Density = (total mass)/(total volume)
Usually density is calculated in a given amount of volume, like cm^3 and the mass used in the equation is the mass that a cm^3 of material weights
so the correct option is mass