This book is not interesting, but that one is ____. (interesting)

Dear Student,
The possible solutions to your query are as given below:

1. This book is not interesting, but that one is. (leave the blank empty as the sentence is complete and meaningful without writing something in it)
2. This book is not interesting, but that one is interesting/very interesting.

Note: If you are not satisfied with this response, please create another question thread and add the heading/instructions for the related exercise along with the question.

Regards!