She and her nephew enjoying an afternoon at the zoo.
Explanation:
Although the options are not included, there is only one possible answer based on this information. The opportunity cost of Lois using Tuesday to work on her project is that she and her nephew will not be able to enjoy an afternoon at the zoo. Opportunity cost refers to the value of the choice that is not taken. In other words, it refers to the opportunity you are sacrificing in order to take advantage of something else.
