Cual libro es tuyo has mistake in Questions lesson
In translating "which book is your?" using the word selections, tuyo isn't even in the list. I picked tu and it gave it to me wrong. Options were: cual, libro, es, tu, cuanto, cuantos, pregunta, cuales, cuantas, cuanta. Sorry I don't have the accents available on my keyboard.
True, but the response said the correct answer was "cual libro es tuyo" and tuyo wasn't even an option
Good catch then. You can email support through email@example.com. Just let them know what phone and the language pairs for the course, and the sentence. :)
The thing to do, is report it during the lesson. In every lesson, after you answer a question, there will be a link at the bottom that says "Report a Problem". Use that, so that Duolingo knows there's a problem with the question. They don't look at the forums to see if there's problems with the lessons.
Is it supposed to be "Cual libro es" by any chance?
If it is supposed to be tuyo but the option is missing, I suggest submitting an error report "report a problem" under "other". If that option has disappeared, I would contact support (white support button on the left) and let them know.
Thank you for helping to improve the courses :)