When the opportunity cost of increasing production is constant at every level, the production possibility frontier is characterized as:

When the opportunity cost of increasing production remains constant across various levels of output, it indicates that resources can be switched from one good to another without any increasing cost. Under these circumstances, the production possibility frontier (PPF) is depicted as a straight line. This straight-line shape reflects constant opportunity costs, suggesting that for each unit of one good produced, a fixed number of units of another good must be forgone.

In contrast, a concave PPF would suggest increasing opportunity costs as production shifts from one good to another, which is indicative of inefficiency in resource allocation. A convex or hyperbolic shape might indicate that the opportunity costs vary significantly as production levels change, which is not the case if the opportunity cost remains constant. Therefore, a linear PPF is the correct representation for a scenario with a constant opportunity cost, highlighting the straightforward trade-offs between the two goods at all levels of production.