The production combinations that fall directly on the production possibilities frontier (curve) are and efficient. (Use one word to fill in the blank.)

The production combinations that fall directly on the production possibilities frontier (curve) are efficient.

This means that when an economy or producer operates on the production possibilities frontier, they are utilizing all available resources in the most efficient manner, without any waste. Each point on the curve represents a maximum output level for one good given a level of production of another good, making it impossible to produce more of one good without sacrificing the production of another. Operating inside the curve indicates inefficiency, where resources are underutilized, while points outside the curve are unattainable with the current resources and technology. Understanding this concept is crucial for analyzing trade-offs and opportunity costs in economics.