Jojen Reed is killed by an undead, skeletal wight. Which is not a White Walker, which are living, sentient beings, so the answer is wrong. Also, since the wight is unnamed and you're looking for the general name of the species that kills Jojen. So the hint should be "What killed Jojen Reed?" and the answer should be: "A Wight."
Enjoy this quiz. Having to have "A" in front of Wight was a bit tricky. If you are looking for more Game of Thrones Quizzes, check out my page. https://www.jetpunk.com/users/maestertywin