Why is density a derived unit?
Density is a derived unit because it is defined in terms of other units.
Density is measured in
The kilogram is defined as the mass of a platinum-iridium cylinder which is kept in France. This is the only fundamental unit which is still defined by an artefact.
The second is defined at the time take for
hence the kilogram and the metre are SI units and density is a derived unit from these units.