This is an oxidation reaction. The balanced equation is
4Fe + 3O2 -> 2Fe2O3
There are other forms of Iron Oxide that can form, notably FeO
Water acts as a Catalyst, but is usually not consumed in this reaction.
If your choices are limited to Synthesis/Decomposition, then the answer would be Synthesis, because you're combining reactants, even though the end product isn't anything you really want.
4Fe + 3O2 -> 2Fe2O3
There are other forms of Iron Oxide that can form, notably FeO
Water acts as a Catalyst, but is usually not consumed in this reaction.
If your choices are limited to Synthesis/Decomposition, then the answer would be Synthesis, because you're combining reactants, even though the end product isn't anything you really want.