Final answer:
The invention of the Transcontinental Railroad led to the population of western states increasing as it facilitated easier travel and settlement in those regions.
Step-by-step explanation:
The invention of the Transcontinental Railroad (TCR) had a significant impact on population movements within the United States. It is not directly referred to in the question, but I assume that the acronym TCR refers to this transformative innovation in American history. Of the options provided, the most accurate result of the invention of the TCR would be that B. the population of western states increased. The railroad facilitated rapid western expansion and settlement by making travel to the West more accessible and affordable. This, in turn, led to an increase in the population as people moved to take advantage of the opportunities provided by the new lands, including farming, mining, and business prospects.
The Native American population, referenced in option A, tragically declined due to diseases introduced by Europeans and other factors, rather than increased. American borders, option C, were not made safe by the TCR alone, as safety can stem from various political and military strategies. The immigration into the United States, mentioned in option D, continued during the period following the railroad's construction, as the nation received waves of migrants from different parts of the world.