Hvilken is used for common gender words (Hvilken bil er din? = Which car is yours?) Hvilket is used for neuter gender words (Hvilket æble spiser du? = Which apple are you eating?) Hvilke is used for plural words (Hvilke fugle så du? = Which birds did you see?)
Thanks, I now see there is much more info about chapters on the website than in the apps. I wish I've been told in the apps on some point, or better yet, if the apps also included longer introductions.