Dividing the density of the process fluid by the density of ____ gives a dimensionless number called specific gravity.

Specific gravity is a dimensionless number that is defined as the ratio of the density of a substance to the density of a reference substance. The conventional reference substance used in defining specific gravity for liquids is water, while for gases, air is commonly used. Therefore, when the density of a process fluid is divided by the density of water, the result is a dimensionless number that indicates how the density of the fluid compares to that of water.

Using water as the reference for specific gravity is particularly relevant because it has a recognized density of approximately 1 g/cm³ (at 4°C). This makes it straightforward to interpret the specific gravity value in practical applications—if a fluid has a specific gravity less than 1, it is less dense than water, and if it has a value greater than 1, it is denser than water. This is essential in various fields, including chemistry and engineering, for understanding fluid behavior in systems.

While other substances, such as mercury or oils, could also be used for comparative purposes in different contexts, water remains the universal standard for defining specific gravity in most scenarios involving liquids. Thus, the correct choice focuses on this widely adopted reference.