I decided to call my chemistry teacher and he said that possible explanation could be that $HNO_{3}$ is a (nitric) acid, whereas e.g. $KNO_{3}$ is an ionic compound, therefore, it's buildnig blocks are not molecules, but ions.

@Jori: It's because I translated it in English. Again, that's all information that's given, nothing else. The question looks exactly the same as the one given in the book, the only difference is that this one is translated in English.

@GM: I agree. Yes, that's all information that's given in the book.