Answer: quality
Step-by-step explanation:
the production possibility curve is simply defined as a curve that shows the amounts of two goods or services that an economy can produce when the available resources are utilized.
It should be noted that when the quantity of a good or service is increased, it affects the other one and there'll be a reduction in the other good. Also, it shows how efficiently the resources that such economy has can be used. Furthermore, it represents scarcity and growth.
A shift of the production possibility curve also means that there's either an increase in economic growth or a decline in the economic growth.
It should be noted that the PPC doesn't represent quality.