Answer:
The correct answer is establishing borders with Canada and Mexico
Step-by-step explanation:
This is the correct answer because United States already had established borders with these countries earlier, mostly in the 19th Century. That is why during the war United States had no problem with this countries, although Germany tried to engage Mexico in the war against the States.
On the other side all other mentioned answers were challenges for the United States. Especially when it comes to trade with Central Powers, as United States were more supportive to the causes of the Allied powers.