the best measure of the opportunity cost of any choice is:

the monetary cost of that choice.
whatever you have given up to give that choice,even if no monetary costs are involved.
the cost associated with not taking full advantage of the opportunity offered by that choice.
none of the above

i would go with b)

The best measure of the opportunity cost of any choice is whatever you have given up to make that choice, even if no monetary costs are involved. Opportunity cost refers to the value of the next best alternative that you could have chosen but didn't. This can include things like time, effort, resources, and potential benefits that you might have received from the alternative choice. While monetary cost is one factor to consider when determining opportunity cost, it does not capture the full extent of the trade-offs involved in decision making. Therefore, the most accurate answer would be "whatever you have given up to make that choice, even if no monetary costs are involved."