So I normally use the word Climax but I'm assuming that climax and main action are the same thing. The main action is definitely (B) - Mary's husband tells her that he is leaving her, because that is what starts the suspense.
Please tell me if that isn't right, I think it is.
Normally the cover, or the short summary on the back of the book
What paragraph? Well immune means protected from something or like you can’t be effected by something