What is a chemical property of matter?

A chemical property of matter is indeed a characteristic that can only be observed during a chemical reaction. This is because chemical properties describe how a substance interacts with other substances, which results in a change in its chemical composition. For example, the ability of iron to rust when exposed to oxygen and moisture is a chemical property; it can only be seen when iron reacts chemically with another substance, leading to the formation of iron oxide.

In contrast, other properties mentioned are related to physical observations or conditions. Observable characteristics without altering the substance are physical properties, and a property measured solely in the solid state might not reflect the behavior of the substance in other states. Additionally, properties that are constant under all conditions generally refer to physical properties as well, which do not change regardless of external conditions. Thus, the emphasis of chemical properties on changes resulting from reactions is what makes the correct answer valid.