Answer:
D. Neutrality
Step-by-step explanation:
Mostly, it was political. Today, most of us are unaware that the Republican Party was vehemently isolationist before to Truman. The Republican party's fundamental concept was that the US should "tend its own garden" and remain within its borders. Republicans' faith in isolationism was as steadfast then as it is now. One cannot overstate how strongly they clung to this notion. The memory of World War I was another factor. Many Americans thought that America's involvement in that war had been a tremendous mistake by the 1930s. The two effectively blocked American involvement in the conflict.
Roosevelt did not fail to involve the US in the conflict in a subtle or not so subtle manner. English participation in the game was aided by American commodities transported in American ships and protected by Lend Lease battleships. To give China a fighter arm, Roosevelt covertly made arrangements for Claire Chenault to pilot the AVG. But the going was difficult. Republicans and a few Democrats battled him from start to finish.
EDIT: I made a typo above; Eisenhower, not Truman, was the Republican Party's isolationist leader. Eisenhower was infamously apolitical and didn't truly belong to any political party for the majority of his life. When it became apparent that "Ike" may win the presidency, he had to decide which party to join. I've read in a few different places that Ike demanded the GOP abandon its isolationist stance in exchange for agreeing to be a Republican. At that point, Republicans had not been inside the White House in nearly twenty years, so isolationism was out the window. In a book I recently read, it was said that Roosevelt had recommended to Ike to switch to the Republican Party for this precise reason just before he passed away. It would totally fit with Roosevelt's character, but I think that's a reach.
Thank you,
Eddie