A rational decision maker takes an action if and only if:

• A. the average benefit equals the average cost
• B. the total benefit of the action is less than the total cost
• C. the opportunity cost is zero
• D. the marginal benefit of the action exceeds the marginal cost

Answer: (D)

Verified step by step guidance

1. Understand the concept of rational decision making in microeconomics: a rational decision maker chooses an action if the additional benefit from that action is greater than or equal to the additional cost incurred.
2. Recall the definitions: Marginal Benefit (MB) is the additional benefit received from consuming or producing one more unit of a good or service, and Marginal Cost (MC) is the additional cost of producing or consuming one more unit.
3. Recognize that the decision rule involves comparing marginal values, not average or total values, because decisions are made at the margin.
4. Formulate the decision criterion mathematically as: \(\text{Take action if } MB > MC\); if \(MB = MC\), the decision maker is indifferent; if \(MB < MC\), the action should not be taken.
5. Conclude that the correct condition for a rational decision maker to take an action is when the marginal benefit of the action exceeds the marginal cost.