6. In the context of the production possibilities fro frontier, opportunity cost refers to amount attached to a good or service. the value of the next best alternative cost of not expanding production cost of labor when production is expanded or changed.

## Step 1<br />The production possibilities frontier (PPF) is a curve that shows the maximum possible output combinations of two goods or services an economy can achieve when all resources are fully and efficiently utilized. <br /><br />## Step 2<br />Opportunity cost, in the context of the PPF, refers to the value of the next best alternative that is given up when a choice is made to produce one good over another. <br /><br />## Step 3<br />This means that when an economy decides to produce more of one good, it has to give up producing some amount of another good. The opportunity cost is the value of that amount of the other good that is given up. <br /><br />## Step 4<br />Therefore, the correct answer is "the value of the next best alternative".