Answer:
Among the options the answer is option A.
Establishing boarders with Canada and Mexico
Explanation: During the World War 1 there were many challenges faced by USA. A huge economic problem during the war which was caused by the war. So recovering this economic problem was a challenge. Moreover, trading in the Europe was also a challenge. Because there were two part during the war which were allied and central power. But establishing boarder with Canada and Mexico was not a challenge for USA as both of the country was not a threat to them.