Which of the following factors promote corrosion?

Corrosion occurs due to reactivity of metal, presence of air and moisture and strains present in metals.
Factors which are promoting corrosion of metal are:
Reactivity of the metal: Metals which are more active in the nature are readily corroded,
Air and moisture presence: Air and moisture present in the atmosphere promotes corrosion. Gases like $S{O}_2$ and $C{O}_2$ catalyze or increase the rate of corrosion.
Strains in the metal: Corrosion takes place easily at scratches, bends and cuts in the metal.