Correct option is C. Hydrated ferric oxide Chemically rust is hydrated ferric oxide. It is formed by the action of atmospheric oxygen gas and water on iron metal. The chemical formula of rust is $$Fe_2O_3$$.
The process of rusting can be shown by the reaction,
$$Fe +O_2 + H_2O \longrightarrow Fe_2O_3$$
For rusting present of water and oxygen gas is essential.