Answer:
The correct answer is option D.
Step-by-step explanation:
A society is assumed to produce two goods, automobiles and tractors. Automobiles are represented on the vertical axis while tractors are represented on the horizontal axis.
As society moves to the right on the production possibility frontier the quantity of tractors increases. At the same time, as the production of automobiles is reduced, the cost of giving up automobiles increases. In other words, the opportunity cost of producing tractors increases.
This happens because of substituting a resource for the production of another good.