"When was this book written?"
Translation:Wanneer werd dit boek geschreven?
I had the same issue with "Wanneer WERD de prins gekroond?" I showed the sentence to a Dutch Language teacher and she said that both "werd" and "was" are correct. Using "werd" just gives the sentence a more passive tone. Bottomline, I do not have an answer to your question, and I am also wondering why "was" is not considered a correct answer here.