What is an externality? a. the highest valued alternative that must be given up to engage in an activity. b. the ability of an individual, a firm, or a country to produce a good or service at a lower opportunity cost than other producers. c. a fixed or variable cost incurred by firms to produce a good or service. d. a benefit or cost that affects someone who is not directly involved in the production or consumption of a good or service. e. the combined benefit and cost to those directly involved in production and consumption of a good or service?