Answer: Option C
Step-by-step explanation: The maximum amount of money that a consumer is willing to pay for having an additional unit of a good or service is called the marginal benefit. Whereas, marginal cost refers to the increase in total cost when one more unit of output is produced.
In the given case, we want to use the available resources at their best, therefore, the money on pollution control should be spent only until the benefit of spending one more dollar is greater than the cost of doing so.
Hence, the correct option is C.