Answer: Option D
Step-by-step explanation: Opportunity cost refers to the cost of of loosing the profits by choosing one alternative over the other.
In the given case, if Celina decides to do her own typing then she will earn $100 in one hour, but at the same time she will loose an income of $200 in an hour.
Thus, the opportunity cost of doing the typing would be more than the benefit she receives from doing so. Hence she should keep working as a lawyer.
Hence from the above we can conclude that the correct option is D.