When iron rusts and forms iron oxide (commonly referred to as rust, which is primarily hydrated iron(III) oxide, Fe₂O₃·nH₂O), the increase in mass can be explained by the fact that the iron combines with oxygen from the environment.
The correct statement is:
"When iron rusts, it reacts with oxygen in the air (or water) to form iron oxide, which results in an increase in mass because the mass of the oxygen that combines with the iron is added to the original mass of the iron."
In this chemical reaction, the total mass of the reactants (iron and oxygen) is conserved, leading to an increase in total mass as the iron combines with the oxygen.