Which measurement expresses the ratio of a liquid's density to the density of water?

The correct choice is specific gravity because it is defined as the ratio of the density of a substance to the density of a reference substance, typically water for liquids. Specific gravity is a dimensionless quantity, meaning it has no units, as it is a ratio of two densities, allowing for a straightforward comparison.

Understanding specific gravity is important in various scientific and engineering applications, particularly in fluid mechanics and material science, as it provides insights into whether a liquid will float or sink when placed in water. For example, a specific gravity less than 1 indicates that the liquid is less dense than water and will float, whereas a specific gravity greater than 1 indicates it is denser and will sink.

The other options do not pertain to the measurement of density relative to water. For instance, mass/volume represents density but does not involve a comparison to water. Pressure measures the force exerted by a fluid per unit area, while molarity quantifies the concentration of a solute in a solution but has no direct relation to density comparisons. Thus, specific gravity is the most fitting choice for expressing the relationship between a liquid's density and that of water.