Final answer:
Flammability is a chemical property which makes it different from boiling point, mass, and thermal conductivity, which are physical properties.
Step-by-step explanation:
The properties listed: boiling point, flammability, mass, and thermal conductivity fall into two categories: physical properties and chemical properties. Boiling point, mass, and thermal conductivity are physical properties because they can be measured or observed without changing the composition of the substance. On the other hand, flammability is a chemical property as it describes a substance's ability to undergo a chemical change, specifically its ability to burn in the presence of oxygen forming new substances.
Therefore, flammability is the property that is different from the rest as it identifies a chemical property while the others are physical properties.