What shape does the production possibilities frontier take when characterized by constant opportunity costs?

When opportunity costs are constant, the production possibilities frontier (PPF) takes the shape of a straight line. This linear relationship occurs because the trade-offs between the two goods being produced remain the same regardless of the output level. In other words, when resources are equally efficient in producing both goods, each additional unit of one good sacrificed will yield a specific fixed amount of the other good.

This straight line represents a constant rate of trade-off, which indicates that the resources involved in producing the goods are perfectly adaptable to both production processes. Thus, as you increase production of one good, you can precisely measure how much of the other good must be given up, resulting in a direct, linear relationship.

In contrast, a curved PPF would suggest increasing opportunity costs, where the trade-offs change as production shifts from one good to another. A step graph would imply that production can only take certain discrete levels, and a circular shape would not accurately illustrate the relationship between two goods in terms of opportunity costs. Thus, the straight line is the correct depiction when opportunity costs remain constant.