Explain opportunity costs using a PPF where investment goods are on one axis and consumption goods on the other.

Again, a good definition of opportunity costs linked to the notion of limited societal resources is the answer. Suppose two bundles of goods - capital and consumption goods - and maximum efficiency in the economy being attained (the economy is producing on the PPF), then any enhance in the production of investment goods will of course mean an opportunity cost in terms of quantity of consumption supplies given up.