Which of these is a chemical property of iron?
A.
It is dense.
B.
It has a melting point.
C.
It is hard to bend.
D.
It becomes rusted.

A, B and C are all physical property. So we cross that out. The remaining option is D. Rusting. Which is a chemical property.

They are all properties of its chemical structure but, if the question is about chemical reactivity, then the answer is D because iron oxidises to form rust. Hope you underastand :)