Zinc acts as a sacrificial anode and corrodes in place of iron, protecting the iron from rusting. This process is known as galvanization, and it is an effective method to prevent corrosion (rusting) of iron. Applying grease can provide some protection, and applying paint can also be a barrier against moisture, but these methods may not be as durable or effective as the application of a zinc coating.
So, the correct option is (C): Applying a coating of zinc.