That question at the end is very awkwardly phrased, and is not written in a way that has a real answer.
The question, after the story, is: "A doctor would report to the police a patient who was a danger to others: True | False | Cannot say"
This doesn't quite make sense as a question. It sounds like it should be asking your opinion, as in: "SHOULD a doctor would report to the police a patient who was a danger to others"
As written, the most generous way to answer it would be that the story tells you that "MOST" doctors believe that they should report such things. So that the answer to the question "A doctor would report.." would be: "most doctors would".
Whether the closest answer in that case would be "cannot say" or "true" is in the eye of the beholder.
Your question:
This one says "a" doctor "would" report. Is this talking about a random doctor or all doctors?
The statement is very unclear and not something a native speaker would write. I would say the closest interpretation would be "a random doctor".
But I suspect that the question was mistranslated, and the best answer to such a question on a test would be "I REFUSE TO ANSWER THIS QUESTION BECAUSE IT IS MALFORMED."