At this point [4], the writer is considering adding the following sentence.
Successful communication is essential if a designer's idea is to become a reality.
Should the writer make this addition here at [4]?
D is the best answer because the proposed sentence to be added is a paraphrase of the sentence before it, containing the same ideas. A, B, and C are incorrect because none fully acknowledges the relationship between the proposed sentence to be added and the other sentences in the paragraph.