Answer:
Technically, A.
Step-by-step explanation:
The real reason the US did not join the League of Nations was because American Progressive Ideals didn't work in Europe due to mulitple socioeconomic factors. So, when congress realized American ideals wouldn't work in Europe, it left the party and league of nations. If u want a basic answer, then the US claimed it wanted to stay neutral in European affairs which to an extent works because America entered the gilded age after WWI which included an increased isolationism to western countries as could be seen in the "Good Neighbor Policy" and outcome of London Conference, which the US also walked out on.
Hope this helps.