D. An author's style may change depending on the author's purpose and audience.
Process of elimination here is helpful for answering this question. First, Option A is wrong because an author's style does not have to be complex to be effective. In actuality, the more complex and author's style is, the less effective the text may become. Option B is wrong because author's style is in every piece of writing. It doesn't have to have characters. A speech, a poem, an essay, and a story all contain elements of an author's style. This is also the reason Option D is correct. An author's style may be humorous and full of puns when telling a story, but can change with a more formal purpose and audience as when giving a speech.
