Answer:
a. law of increasing relative cost
Explanation:
In economics, the law of increasing costs is a theory which states that once all production factors (land, labour, capital) are at maximum output, it will cost more than average to produce.
As production increases, the opportunity cost will also increase.
In such a nut-shell, markets and countries are trying to strike a balance on how to handle resources whilst using the goods to achieve the best possible productivity and revenue.