A production possibility curve is usually concave (bowed out) to the origin because:

Correct answer: A) The opportunity cost of producing more of one good rises as more of it is produced

Explanation

Resources are not equally suited to all uses. As more of one good is produced, resources less suited to it must be shifted, so more of the other good is given up per extra unit - increasing opportunity cost - which makes the PPC concave. Equally efficient resources would give a straight-line PPC.