A. property is a characteristic of a pure substance that can be observed without changing the substance into some thing else

The answer is physical property.

Physical properties are the properties that does not require the substance changing into another substance, which is, chemical change. Physical properties are usually observable and measurable, such as melting points, density, odor, color etc. These does not require a chemical reaction to happen.

The opposite of physical property is chemical property, which is how the substance changes to another. Such as how sodium reacts with water to make sodium hydroxide and hydrogen.