Final answer:
The correct answer is 'B) utilization review.' It is the method by which health plans monitor the usage and necessity of the medical services their members utilize, differentiating from coinsurance or copayments.
Step-by-step explanation:
The method used by a health plan to measure the amount and appropriateness of health services used by its members is called a utilization review. This process involves assessing the medical necessity, efficiency, and/or appropriateness of the health care services provided to policyholders. It contrasts with other financial features of health plans like coinsurance, deductible, and copayment, which are methods to share the cost of healthcare services between the insurance company and the policyholder, aimed at reducing moral hazard. Whereas, the utilization review focuses on managing health care services to optimize patient care and minimize unnecessary spending, which can help counteract both moral hazard and adverse selection in insurance markets.