Opportunity cost measures the cost of any choice in terms of the next best alternative that must be given up. It is essential for rational decision-making, as it forces individuals and firms to consider the hidden costs of their actions beyond just monetary expenditures.

Every society faces scarcity of fundamental resources such as food, shelter, and protective coverings, which are essential for human survival. These basic needs are universal and always in limited supply, driving economic decisions and resource allocation. Scarcity is the central problem of economics, necessitating choices about how to distribute limited resources to satisfy unlimited human wants.

An outward shift of the production possibilities frontier (PPF) indicates that an economy can produce more of all goods than it could previously. This shift is caused by factors such as technological progress, an increase in the quantity or quality of labor, or an increase in the stock of capital, all of which expand productive capacity.

Opportunity cost is the value of the next best alternative foregone when a choice is made. Since resources like time and money are scarce, every decision—whether napping, studying, or watching sports—necessarily involves giving up the opportunity to do something else with those resources.

Economists use production-possibility schedules or curves to map the boundary of possible output combinations given the limits of resources and technology. This model helps visualize trade-offs, efficiency, and the concept of opportunity cost, as producing more of one good necessitates producing less of another.

A production possibility frontier (PPF) with constant opportunity costs is represented by a straight line. This indicates that the rate at which one good can be traded off for another remains unchanged regardless of the level of production. This occurs when resources are perfectly substitutable between the production of two goods, meaning the marginal rate of transformation is constant throughout the entire range of production.

An economist acts as a scientist when they formulate and test hypotheses about how the world works using positive analysis. Option A is a testable, positive statement about the relationship between policy and labor market outcomes. The other options are normative statements that prescribe policy based on subjective moral or social goals.

Normative economics involves value judgments and opinions about how the economy 'should' or 'ought to' function. Option D expresses a policy recommendation based on subjective priorities, whereas the other options are positive statements that can theoretically be tested or verified with empirical data.

Positive economics focuses on objective analysis and factual descriptions of the economy. These statements are descriptive and can be empirically tested or verified against data to determine if they are true or false, unlike normative statements which are based on subjective value judgments.

Normative statements express value judgments or opinions about what 'should' be, rather than describing facts. Option D is normative because it asserts a subjective priority regarding social issues, whereas the other options are positive statements that can be empirically tested or verified with data.