Final answer:
PPFs are usually bowed outwards because the shape reflects the law of increasing opportunity costs, indicating that more resources are needed to produce additional units of a good as production rises. The correct option is option (a).
Step-by-step explanation:
The reason PPFs (Production Possibility Frontiers) are usually bowed outwards is because this shape reflects the law of increasing opportunity costs. As more of one good is produced, the opportunity cost of producing that good increases, meaning that to produce additional units, society must give up increasingly larger amounts of other goods.
This is because resources are not equally efficient in producing every good, so as resources are reallocated from the production of one good to another, the productivity diminishes. The law of increasing opportunity costs gives the PPF its concave shape, indicating that the cost of reallocating resources from one product to another becomes higher as you continue to increase production of that product.
The law of diminishing returns also contributes to the bowed-out shape of the PPF by illustrating that as additional units of resources are devoted to a particular production, the additional output created by those units will eventually decrease. Therefore, the slope of the PPF starts relatively flat (reflecting lower opportunity cost) and becomes steeper (reflecting higher opportunity cost) as more of a single good is produced and resources are moved from their most productive uses.