Answer:
A) Reactivity
Step-by-step explanation:
Reactivity is one of the characteristics of a chemical property. The copper that turned color due to air, has a different chemical composition.
It wouldn't be flammability - even though it is a chemical property, but nothing is burning.
All the other answer choises: mass and volume are physical properties. Basically the PHYSICAL characteristics which doesn't change the composition.
Hope this helps!