I think its 3 precipitation.
The reaction of a metal with oxygen to form rust is called as redox reaction.
Explanation:
The process during which the surface of metal reacts with the oxygen ions present in air, it has tendency to get oxidized that is formation of an oxide layer on the surface of the metal. Generally, it occurs when metals like iron comes in contact with moisture or water. So the oxygen in moisture reacts with iron giving its electron and getting oxidized and reducing the surface of metal by forming metal oxide. So the complete process of formation of rust is termed as redox reaction. As here both oxidation and reduction is taking place.
