The best response to explain why the rusting of iron is a chemical reaction is:
Iron and oxygen atoms rearrange themselves to form iron rust. Iron rust did not exist before the reaction.
This response correctly identifies that during the rusting process, the atoms of iron and oxygen undergo a chemical change, resulting in the formation of a new substance (iron rust, Fe2O3) that did not exist prior to the reaction. Chemical reactions involve the breaking and forming of bonds between atoms, which leads to the creation of new substances.