"It is your book."
Translation:Het is jouw boek.
Using "je" as a possessive (instead of jouw) is indeed possible, if slightly informal and more common in spoken Dutch. It would change the meaning of this sentence, by removing the emphasis from "your". When you say "Het is je boek", the ownership of the book is not being discussed.
The multiple choice gave me the following choices:
Het is uw boek.
Het is jullie boek.
Het jongen jouw boek.
I knew the last one should not have "jongen" in it instead of "is", so I picked the top two choices and lo and behold I was correct???!!!! Is that really correct?