Final answer:
The PPF graph demonstrates the need to make choices about resource allocation due to scarcity, which leads to opportunity costs.
Step-by-step explanation:
The question refers to the production possibilities frontier (PPF), which is a graph that represents the trade-offs involved in allocating resources between the production of two goods or services. The PPF slopes downward and to the right, indicating that there are limited resources and, because of that, producing more of one good or service means producing less of another. In economic terms, the PPF illustrates the concept of opportunity cost, which is the value of the next best alternative foregone when making a choice. Therefore, the blank in the question should be filled with the phrase "opportunity costs". The downward slope of the PPF shows that resources are scarce and therefore we must make choices about how to allocate them efficiently.