Answer:
Option B. Combustion
Explanation:
An iron fence reacts with with the oxygen in the air to form rust.
In this process iron combines with oxygen of the air forming rust. The formation of rust is an oxidation-reduction with the release of energy.
Rusting of iron is an exothermic process as heat is released during rusting process.
Therefore, it's a kind of slow combustion reaction.
Option B. is the answer.