What is the best definition of specific gravity?

Specific gravity is defined as the ratio of the density of a substance to the density of a reference substance, typically water for liquids and solids. The correct choice highlights that specific gravity specifically pertains to the density of liquids in relation to water. This measurement provides insight into whether a liquid will float or sink when placed in water, as 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 suggests that the liquid is denser and will sink.

In the context of the other options, they do not adequately capture the essence of specific gravity as it is traditionally measured. For example, while analyzing the density of solids compared to water is relevant, it doesn't fully encompass the definition as specific gravity primarily involves liquids when discussing practical applications. Similarly, the comparison of the weight of a liquid to air deals more with buoyancy rather than specific gravity. Additionally, the effect of temperature on liquid density, while important in various contexts, does not define what specific gravity is. Thus, the focus of specific gravity on liquid density relative to water clearly makes the second option the best definition.