The correct answer would be: It clearly explains the book's topic.
Even though this paragraph may show an interest “hook” to attract readers attention, it is completely devoted to explain the books topic. The first sentence more than a hook is telling what readers will find in the book and the following sentences explain very clearly what the author will talk about and what readers will find through the book.
"It adds credibility to the author" or "It is written by an expert on the subject" are not the correct answers because, even though you can assume the experience on the topic of the author in the words and how the author describes dancing genres, there is no specific statement that proves a solid backup in his or her knowledge (supporting these two statements).