Shouldn't the answer the question be $0. Because you arent really giving up anything?
Opportunity cost includes the cost of whatever good is being purchased, but also the cost of whatever alternative is given up in order to acquire the chosen good. In this example, it would include the $100 spent on purchasing services from the dentist, and nothing else because Wanda does not have to leave work early and the question specifically states that she normally does nothing after work.
If she did have to leave work early then she would be giving up wages that she could have earned and that would have to be added to the $100 to obtain the opportunity cost.
Also, if there were information given about how Wanda values leisure then she may be giving up leisure and that could be included in the opportunity cost. in this case it is clear that you are to assume no cost of losing leisure.