Which of the following is not considered a physical property of matter?

Flammability is classified as a chemical property of matter. It describes how a substance reacts with oxygen and the conditions under which it can ignite, which involves a change in the substance's chemical composition. In contrast, the other options—density, melting point, and boiling point—are all physical properties. Physical properties are characteristics that can be observed or measured without altering the substance's chemical identity.

Density refers to the mass per unit volume of a substance, while melting point is the temperature at which a solid becomes a liquid, and boiling point is the temperature at which a liquid becomes a gas. All these properties can be observed or measured without changing the material itself, highlighting why flammability stands apart in this classification.