Final answer:
The best paraphrase for the passage on the Pony Express's significance in paving the way for the transcontinental railroad is Option A. It accurately conveys the historical impact of the Pony Express on American rail infrastructure development.
Step-by-step explanation:
The passage discusses the significance of the Pony Express in demonstrating the feasibility of a transcontinental railroad before the completion of the Pacific Railroad. It specifically highlights that the Pony Express proved that a year-round operational transcontinental line was possible, a task once believed to be impossible. The best paraphrase of the passage is Option A: "In a feat that had always been regarded as impossible, the Pony Express proved the possibility of a transcontinental railroad and demonstrated that such a line could be built and operated year-round." This paraphrase captures the essence that the Pony Express was a precedent to the later development of rail infrastructure that would revolutionize American travel and commerce.