The best explanation of the political role of the U.S. during World War I is:
Although the U.S. joined the war late, it played an influential role in the peace process.
This statement highlights that while the U.S. did not participate for the entirety of the war, its involvement in the latter stages of the conflict and the subsequent negotiation of the Treaty of Versailles played a significant role in shaping post-war politics and the structure of international relations.