After analyzing the options containing different parts of a book, we can say that the author would explain why she wrote that book in:
A. The introduction
First, it is important to understand that the correct place to explain the central idea of a book and why you have written it is in the preface. However, preface is not one of the answer options provided.
For that reason, we will choose introduction, and here is why:
With that in mind, the only good option to answer this question is B. introduction. This is where the author will present her ideas to readers.
Learn more about introductions here:
https://brainly.com/question/1047087