Opportunity cost is A. the highest valued alternative that must be given up to engage in an activity. B. the ided that because of scarcity, producing more of one good or service means producing less of another good or service. C. when consumers and firms use all available information as they act to achieve their goals. D. when unlimited wants exceed the limited resources available to fulfill those wants.

To determine the correct definition of opportunity cost, let's analyze each option: A. **the highest valued alternative that must be given up to engage in an activity.** - This definition accurately describes opportunity cost. It emphasizes that when you choose one option, the cost is the value of the next best alternative that you forgo. B. **the idea that because of scarcity, producing more of one good or service means producing less of another good or service.** - This statement relates more to the concept of trade-offs and the production possibilities frontier rather than specifically defining opportunity cost. C. **when consumers and firms use all available information as they act to achieve their goals.** - This describes rational decision-making but does not define opportunity cost. D. **when unlimited wants exceed the limited resources available to fulfill those wants.** - This statement describes the concept of scarcity, not opportunity cost. Based on this analysis, the correct answer is: **A. the highest valued alternative that must be given up to engage in an activity.**