The text that clearly has an agenda is a book about how to find success, which the reader can start doing by buying the author’s other books on the subject (A).
This text most clearly has an agenda because it immediately promotes the author’s other works on the subject of finding success. It seems that the author’s goal is more focused on selling his or her products instead of genuinely wanting to help the reader find success. This suggests that the book was created for a more commercial purpose rather than the exploration of the initial topic.