Complex sentence: In microeconomic theory, the opportunity cost of a choice is the value (not a benefit) of the best alternative forgone where, given limited resources, a choice needs to be made between several mutually exclusive alternatives.
Split into simpler sentences: In microeconomic theory, the opportunity cost of a choice is the value (not a benefit) of the choice of a best alternative lost while making a decision. A choice needs to be made between several mutually exclusive alternatives.