The opportunity cost was: The money she would have earned working.
Explanation:
When there exists two choices and you choose one, then the profit that you lose when selecting one choice over the other choice is called as the opportunity cost. Thus it helps in choosing a best decision where there are more alternatives are available.For instance consider you have 100$ and you decide to invest either in useful things or something that you enjoy.
In the example given, Sam is offered with an extra shift where she gets an extra pay. But she told to give that offer to some other person. Hence, the opportunity cost will be the amount that she might have earned when she accepted to work night.
