Final answer:
Economic decline is represented on a production possibilities frontier model by the production possibility frontier shifting inward.
Step-by-step explanation:
The correct answer is C) Shifting inward. Economic decline (negative growth) is represented on a production possibilities frontier model by the production possibility frontier shifting inward. This means that the economy is producing less than it is capable of producing due to factors such as decreased resources, technology, or productivity.
For example, during a recession, businesses may downsize, reducing their production capacity and causing the production possibilities frontier to shift inward. This indicates a decrease in the quantity of goods and services that the economy can produce.