I believe the answer is A) The cold war.
I hope this helps :)
US foreign policy today is hard to describe as its changing with the election of trump but lets say that we were observing the time a bit before this and more in the present-day era.
We used to stay neutral will all countries we could but we began to see that it was not always best to stay neutral (as in the case of WWI and WWII). The failed attempt after World War I to make a league of nations that operated well just demonstrated the problems we had, and was the reason why WWII happened and adolf hitler was able to come to power.
Today, we use economic and military forces on places that are labeled unstable to try and make these places more stable and set up a democratic government.
This for the most part, has failed miserably such as in Syria and Iraq.
Again, mods, please write me if you have concerns over links but I find students learn best sometimes from visuals. This video describes the problem that happened in Iraq/Syria that led to the failure of US troops trying to make the government more stable.
https://www.youtube.com/watch?v=AQPlREDW-Ro
This second video goes more in-depth about the Syrian crisis and shows how US foreign policies affected it.
https://www.youtube.com/watch?v=RvOnXh3NN9w&t=12s
Frederick Jackson Turner was an American historian that defended that the strength and the vitality of America’s identity lay in its land and vast frontier.
He said that the vast frontier was very important for an - then young - country like America. He also said that America should always seek new frontiers abroad.
This changed people’s idea and America identity, this led the US to call for new frontiers like in Cuba, Philippines, Puerto Rico, and Guam.
Because of this growing frontiers, America became a diverse country with different countries and tolerant social mix, America is a safe place for many races and religions, all because of its frontiers.
At the Berlin Conference the principle of Effective Occupation was established, that said that a country needed to have an actual presence in the colony to be bale to claim it.
In practice, this meant at least <span>c. establishing businesses there. Additionally, they could make contact and conquer the local rulers. </span>
During the 1790s, the primary goal of United States foreign policy was to expand westward by signing new military alliance
Hope this helped :D