Answer:
D) México
Explanation:
The answer is D Mexico because the text explicitly mentions that many Mexicans crossed the border into the United States during the war. It states that Mexicans were living in poverty in Mexico and sought better lives for themselves and their children. They immigrated to the American West and found work on large ranches and farms. This increased immigration from Mexico was a result of the demands of the war and the need for labor in the agricultural sector.