Specific gravity is best defined as:

Specific gravity is commonly defined as the ratio of the density of a substance to the density of a reference substance, typically water, at a specified temperature. This measurement provides insight into how dense a material is compared to water. When a substance has a specific gravity less than one, it will float in water, while a specific gravity greater than one indicates that it will sink.

Using water as the reference is particularly useful because it has a density of approximately 1 gram per cubic centimeter (g/cm³) at standard conditions, making calculations and comparisons straightforward. This relationship helps in various fields, including engineering, chemistry, and physics, allowing for the assessment of material properties relative to one of the most abundant and commonly encountered substances.

The other options do not accurately define specific gravity. Weight pertains to the gravitational force acting on an object and does not consider the concept of density. Density of liquids in relation to air does not focus on the typical reference of water used, and the pressure of a liquid refers to the force exerted by the liquid per unit area, which is unrelated to the concept of specific gravity. Thus, the definition that centers around the density of liquids relative to water is the most accurate understanding of specific gravity.