The correct answer is: D
The central idea is the most important content of a work, a proposal, a project, etc. Without that central idea, it would not be understood or lose its value. For example: "The central idea of Little Red Riding Hood is that you should not disobey your parents".
It can be said that the central idea is the most important of a text or other manifestation of thought. If we take the concrete case of the texts, we will notice that they are composed of different thoughts or ideas. Many of these ideas are secondary or accessory: they help to create a context and to reinforce the main thing, but they can be dispensed with without changing the meaning of the text. The central idea is the basis that the author maintains and that allows him to tell what he wants.