The best answer is:
C) a choice that must be made due to scarcity.
A tradeoff occurs when you must choose between two or more things, selecting the best option given the constraints. Choosing what to spend your allowance on, for example, is involves a trade-off that you must make due to the scarcity of your allowance money (you don't have unlimited money). D is a tempting answer, but it does not define trade-off as well as C.
