Answer:
Volume.
Step-by-step explanation:
Concentration is defined as the ratio between the amount of substance in terms of mass units or moles and the volume or mass of a solution. There are many ways to look at it.
1. Molarity: it's the measure of concentration expressed as moles of solute divided by the volume of the solution in liters (mol/L units, or M);
2. Molality: the measure of concentration expressed as moles of solute divided by the mass of solvent in kilograms (mol/kg units, or m);
3. Percent mass: the measure of concentration expressed as mass of the component divided by the total mass of the mixture (%w/w, or simply %);
4. Percent volume: similar to percent mass, but in this case we divide the volume of the component by the volume of the mixture (%V/V);
5. Mole fraction: this is the ratio between the moles of component and the total moles of all the components present in the mixture (dimensionless, but might be expressed as percentage if needed).
As you can see, concentration can be expressed very differently. What unites all of these concepts is the fact that we're looking at a fraction of the solute/component with respect to the total mass/volume of the solution or the mixture.
Typically, we're interested in molarity, as it's used most often, that is, the ratio between the moles and volume. Remembering that moles is directly proportional to mass (moles are equal to mass divided by molar mass), we may also measure concentration as mass/volume fraction instead of moles/volume.