The correct answer is B.
A trade-off is defined as a decision between two options, in which, in order to select one of the alternatives, the other is lost. It is a widely-used term in economics.
The necessity of facing such trade-offs comes from the fact that economic resources are scarce. Each consumer, given a fixed endowment of resources, has to decide on which goods he desires to spend his limited income, provided that his endowment cannot afford them all. Therefore, deciding in economic implies renouncing to the non-selected alternatives (opportunity cost).