The correct answer is: D) when the quotation expresses the idea better than the essay writer would be able to.
Sometimes the things we think and want to state had been already said by an author, that author was probably an expert on the subject and developed the idea in a very advanced level, that is the reason we quote him/her. By paraphrasing, sometimes the meaning of the things expressed in the quote are lost, as a result, the effect we wanted to produce in the audience is not deep or powerful enough. Some writers posses such a capacity to sinthesize subjects or to describe things that it becomes neccessary for us to quote them. To quote, the author must have a deep undestanding of the author quoted, in this way the quote can be used in a proper manner to give flow and sense to the text.