Final answer:
The production possibility graph slopes down primarily due to the law of increasing costs and limited resources.
Step-by-step explanation:
The production possibility graph slopes down primarily due to the law of increasing costs, which states that as the production of one good increases, the opportunity cost of producing additional units of that good also increases. This happens because resources are not infinitely available, and as more resources are allocated to producing one good, there are fewer resources available for producing other goods.
Additionally, the graph also reflects the limited resources available in the economy. The negative slope of the graph indicates that producing more of one good requires taking resources away from producing the other good, demonstrating the concept of scarcity.
The other options mentioned, such as inefficiency, improper output mix, and unemployment of resources, can also impact the production possibility graph, but they are not the primary reasons for the downward slope.