A production possibilities frontier (PPF) with a bowed outward shape indicates c) increasing opportunity costs as more and more of one good is produced.
This means that as you allocate more resources to produce one good, the amount of the other good you can produce decreases at an increasing rate. The reason for this is that resources are not perfectly adaptable for the production of both goods. For example, if you are producing wheat and you start to use land that was previously used for corn, the initial land may be very suitable for wheat, but as you keep reallocating land to wheat production, you will start using less suitable land for corn. Hence, the opportunity cost of producing additional wheat rises.
In contrast, a straight-line PPF would imply constant opportunity costs, showing that resources can be shifted between the two goods without changing efficiency. The bowed shape of a PPF visually represents the reality of resource allocation in an economy, illustrating how choices lead to trade-offs in production.