The question is aimed at assessing the understanding of the progress made in creating the mechanical version of science fiction. The correct answer is 'D. Scientists have begun to come close' because the passage states that scientists have yet to create the mechanical version but have begun to come close. Option A is incorrect because the passage indicates that the mechanical version has not been fully realized. Option B is incorrect because the passage suggests that progress is being made. Option C is incorrect because the passage does not mention the creation of prototypes or being far from completion.