The correct answer is B. "Fear of becoming involved in another European War."
After World War I, the US wanted to return to its policy of neutrality. This was America's foreign policy up until 1917, when they officially entered World War I. Even though the US did not want to initially get involved, incidents like the sinking of the Lusitania and the Zimmerman Telegram somewhat forced them into this war.
Now that the war was over, the US wanted to return to focusing on just American issues rather than foreign conflicts. This is why the Senate refuses to pass the bill that would allow the US to join the League of Nations.