Reason of passivity of iron is (RPMT 2007)

Passivity is a state of inactivity of metal that is observed due to the oxide layer on the surface. When a piece of iron is dipped in concentrated nitric acid and then allowed to react with other reagents, iron loses its activity and shows inertness. It acts as a passive metal. This is seen due to the formation of an oxide layer on the iron surface. The reason for the passivity of iron is due to the 3 4 Fe O layer. Thus, (b) is the correct option.