Answer: The correct answer is "A. rent the room because the marginal benefit exceeds the marginal cost.".
Explanation: If the marginal cost of operating one room for one night is $30 and the customer is willing to pay $60 for the night, the hotel manager should rent the room because the marginal benefit exceeds the marginal cost.
The hotel manager should always rent the room while the marginal benefit is greater than or equal to the marginal cost.