ANS:D - all (a), (b) & (c).

The correct statement is: Sacrificial anode occurs higher in the electrochemical series of metals. Sacrificial anodes are made of metals that are more active (i.e., higher in the electrochemical series) than the material they are protecting. This ensures that the sacrificial anode corrodes preferentially, sacrificing itself to protect the base material from corrosion. The other statements are also true: Sacrificial anodes can be made of materials such as magnesium, aluminum, or zinc, which are more active than steel or iron commonly used in pipelines. Sacrificial anodes are typically cheaper than the material they protect because they are made of less expensive metals and are designed to be consumed over time as they corrode, requiring periodic replacement. So, the correct answer is: all (a), (b) & (c).