The correct answer is c) negatively sloped straight line.
When we talk about constant opportunity costs, we mean that the trade-off between two goods remains the same no matter how many of either good we produce. In this situation, the production possibilities frontier (PPF) is represented as a straight line that slopes downward from left to right.
This negative slope indicates that to produce more of one good, you have to give up some of the other good at a constant rate. The straight line reflects that the resources are perfectly adaptable for the production of both goods, leading to unchanging opportunity costs. Thus, the more you produce of one good, the same amount of the other good must be reduced, resulting in a linear PPF.