To determine what Capeland gives up by switching from producing 0 watermelons to producing 18 million tons, we need to know the opportunity cost of producing watermelons, which is measured in terms of the alternative good that could have been produced instead (in this case, shoes).
Given the options provided, we don't have specific information about the trade-off between watermelons and shoes from the problem statement. However, you can generally determine that to find the most accurate answer, you'd ideally need a production possibilities frontier (PPF) or data about how many shoes are produced at 0 watermelons and at 18 million tons of watermelons.
Since none of this information is provided, it's impossible to state definitively what Capeland gives up without further context about its production capabilities. If we assume that the options represent possible quantities of shoes that could be produced instead, we can make a general interpretation.
Without additional data, it is not possible to provide an answer. However, if you have specific information about the trade-off rates for Capeland, please share that, and I would be happy to help calculate the opportunity cost.