D is the answer to this question. The Rich Text Area is known to be an interface wherein you can edit the rich text that you have used in order to create your website. This means that you will be able to see how the text would appear and how the text would actually look like once it is already on the website.
You can also format the text with different options which you believe can help improve the appearance of the text. For example, if you need to highlight some of the text, this is the area wherein you can do this. Plain text will contain no formatting and no changes in the font, font size, and even the color of the text used.