In 1860, a person could travel only as far as the Missouri River by rail from New York. Option C is correct.
After the Transcontinental Railroad was completed, the train could go all the way to the west coast. Transcontinental railroad was a major development due to the fact that it allowed continuous railroad for more than 3000 km.
A transcontinental railroad constitutes a contiguous network of railroad trackage that crosses a continental land mass with terminals at different oceans or continental borders.