The correct answer is A. Oxidation.
Rusting is a form of oxidation, which is the chemical reaction of iron with oxygen in the presence of water. This reaction produces iron oxide, which is the red-brown substance that we see as rust.
Carbonation is the reaction of carbon dioxide with water to form carbonic acid. This acid can dissolve limestone and other carbonate rocks, but it does not react with iron.
Exfoliation is the peeling or flaking of a material, such as paint or metal. It is not a chemical reaction, but rather a mechanical process.
Corrosion is the deterioration of a material due to chemical or electrochemical reaction with its environment. Rusting is a form of corrosion, but it is not the only type. Other types of corrosion include pitting, stress corrosion cracking, and galvanic corrosion.
In conclusion, rusting of iron takes place due to oxidation. This reaction produces iron oxide, which is the red-brown substance that we see as rust.