What property of a substance is defined by its density?

The correct response identifies density as the mass of a substance divided by its volume. This fundamental property can be represented mathematically as density = mass/volume. It allows us to understand how much matter is contained in a given space and is crucial in distinguishing between different materials, especially when they may appear similar.

Density is a key concept in various scientific applications, such as buoyancy, where an object's ability to float or sink in a fluid is determined by its density relative to that fluid. It also plays a significant role in defining the behavior of gases, liquids, and solids under different conditions.

While the other options address different physical properties (weight is related to the force of gravity on an object's mass, molecular size refers to dimensions at a molecular level, and temperature measures thermal energy), they do not define density in the way that mass per unit volume does. Understanding density is essential for applications ranging from material science to engineering and environmental science.