Technically, yes "Unknown" is the correct answer for how Thresh died, because we are not specifically told HOW he dies. Because of this, I did get the right answer. However, I think it's safe to presume his death was due to Cato or the Mutts, so you should accept these answers as well. I doubt he tripped and fell on a particularly sharp branch.
Also, like mariofan says, a lot did happen to Peeta in Mockingjay. Yes, his being Hijacked is the main event, but other stuff happened. Perhaps "What did the Capitol do to Peeta?" would be a better worded question.
And no, I don't agree that 'brainwashed' should be accepted for this answer. Brainwashed is more using methods such as hypnosis to make someone believe something, whereas with hijacking you are phsyically taking out someone's memories and replacing them with false ones. Besides, Suzanne Collins uses the word 'Hijacking' throught the book.