Your answer should be ‘the rapid settlement of the American West’, since according to senate.gov, it reads ‘ | The rapid settlement of the western territories | in the 1850s convinced most members of Congress of the need for efficient rail transport to the Pacific coast,’.
Trade route from Europe to China had always been a land route before the discovery of sea route in 15th century.
There are multiple reasons people didn't want to go to Vietnam. Firstly, it was immoral and people didn't like the fact we were going to a third world country and massacring it's people. Many also thought that the whole mission lacked a clear objective and appeared unwinnable.
TL;DR - It's probably answer choice A