I think this has to do with parallelism, correct me if I am wrong.
The main idea
Readers of the book said it is at once frustrating because ________
so you want an answer that provides a counter reaction using the same format, 'adjective' because ________
So the only choice that follows this format is E
and delightful because_______
frustrating and delightful are both adjectives describing the book. With choice E, they both provide an adjective and both tell why using because
Readers of the book said it is at once frustrating because _________ and delightful because ______________
Now you might have been expecting to use a word like but, although, or however, but think about it this way, 'readers say the book is _____ and _____'
readers are saying that the book is two things, frustrating and delightful, not frustrating while its delightful.
Thats how I look at it
