Which of the Following Is the Best Definition for Opportunity

Which of the following is the best definition for opportunity cost?

A) Depreciation method under Canadian tax law allowing for the accelerated write-off of property under various classifications.
B) A cost that has already been incurred and cannot be removed and therefore should not be considered in an investment decision.
C) Evaluation of a project based on the project's incremental cash flows.
D) Financial statements projecting future years' operations.
E) The most valuable alternative that is given up if a particular investment is undertaken.