The correct option is C The difficulty of travel
The difficulty of travel.
First, decode what the question is asking: which factor is the author referring to? Basically, we’re being asked to figure out what the word “this” refers to. Read the sentence before the phrase referenced: “Travel by ship was more expensive, but the terrain in what is now the western United States was so harsh and the conditions so perilous that the average journey time by either land or sea was six months.” With that in mind, what would make someone want to create a railroad? The difficult travel conditions is the most likely factor, with both land and sea travel cited here. This makes 3 the best answer.