We know that the density is defined as the ratio of mass to the volume.
Density = Mass/volume
We know that the unit of the mass is kg
Unit of the volume is liter (l)
Therefore the unit of the density will be kg/l .
Now the first expression is 5.2 Cm^3 which is a unit of volume. Therefore it will not be the expression for density.
Second expression is 0.71 mg which has unit of mass. Therefore it will also not be the expression for the density
Third expression is 20 g/cm^2 which also not have the unit of density.
Fourth expression is 5.13 kg/l which is having unit of density.
Therefore the expression of density will be 5.13 kg/l