I'd say it's D) Indirect object, but I'm not sure, so don't take my word as gospel :)
Answer:
That they have done so for their own sake
Explanation:
The author may describe reading as a positive escape because of their own personal experiences. Books appeal to the readers' emotions as to draw them into the book. Reading can be positive because it betters your brain, and can help you experience things inside of your imagination.
