For these questions regarding finding the main point, it is typically easiest to try to zone in on what the author is driving at. What is the final point they are making that all of the evidence they use supports. This can be facilitated by use of conclusion words (such as therefore, hence, so), counterpremise words (such as but, however, yet).
Her point here is that Marcus' principle is not absolutely correct and without exception. She uses the example of a journalist in a quandary about whether the news is important/newsworthy. She claims this is a typical case.
That is how we get to the conclusion provided in answer choice C.
I hope this helps. Please let me know if you have any other questions or would like me to elaborate further!
