The answer is D.
In both speaking and writing, you are trying to convey a message. Even if you are just telling a story, or writing a fictional novel, you then want to take your audience to this made up place. You need details, but more importantly, you need words that are best suited to your audience. You have to be precise. If it's a younger audience, you wouldn't use complicated words, but if you're talking/writing about the subject of your phd, you're going to use a more technical language.
