Final answer:
Reactivity is a chemical property because it describes a substance's ability to change chemically, which cannot be observed without altering the substance itself.
Step-by-step explanation:
Reactivity is a chemical property of matter because C. reactivity cannot be observed without changing the substance. A chemical property describes the ability of a substance to undergo a specific chemical change. To identify a chemical property, we look for a chemical change, which results in matter with a different identity than it had prior to the change. Chemical properties can include a wide range of behaviors, such as flammability, toxicity, acidity, and various other types of reactivity. For instance, iron combines with oxygen in the presence of water to form rust, indicating its reactivity with oxygen, whereas chromium does not easily oxidize. This illustrates that the observation and measure of reactivity inevitably involves altering the substance's chemical structure or composition.