Which of the following best describes a chemical property?

A chemical property is fundamentally concerned with the ability of a substance to undergo a chemical reaction and form new substances. In this context, the ability to react with oxygen is a prime example of a chemical property because it indicates how a substance can change its chemical structure when exposed to another reactive element, leading to phenomena such as combustion or oxidation.

The other options refer to physical properties. The appearance of a substance pertains to its visual characteristics and physical state and does not involve any change in chemical composition. Density describes how much mass is present in a given volume of a substance, which also does not result in a chemical reaction. Similarly, the melting point of a substance is a measure of the temperature at which it transitions from a solid to a liquid, a physical change rather than a chemical one. Therefore, when identifying a characteristic that reflects the substance's capability to undergo a chemical change, the ability to react with oxygen stands out as the correct description of a chemical property.