Which of the following is NOT a method of preventing the corrosion of iron?

Paint or grease is a barrier that keeps out air and moisture. Galvanisation (zinc coating) gives both barrier and sacrificial protection. Connecting iron to the negative terminal of a DC source makes it a cathode (impressed-current cathodic protection), so it cannot be oxidised. Connecting iron to a less active metal like copper makes iron the anode of the couple and accelerates its corrosion, so (D) is not a protective method.