Answer: The correct answer is "smaller".
Explanation: The principle of increasing marginal opportunity cost states that the more resources devoted to any activity, the smaller the payoff to devoting additional resources to that activity.
This principle, better known as, the law of diminishing (marginal) returns, establishes that by increasing the amount of a productive factor in the production of the good or service in question, the production yield is reduced as we increase this factor As long as all other factors are maintained at a constant level (ceteris paribus).
It is a marginal decrease, that is, the increase is smaller every time.