The real answer is B. A lot happened in the 20th century. List all the major events or all the events you need to talk about.
1. WWI
2. The Russian Revolution
3. The Great Depression
4. WWII
5. The Death of FDR, Hitler and Stalin later.
6. The Fall of the Berlin Wall. The Berlin Airlift
7. The Rise of Japan
8. Vietnam
9. The Beginning of the Rise of China
All of these except the death of the leaders of WWII were Military and/or Economic Events. All of them left America even stronger militarily or economically. Just go down the list and see why. WWI was the last hurrah of the power of the European nations and the victors squandered their victory.
Russia's revolution led a great and daring experiment. No one had ever tried anything like that before. Could man survive a planned economy. The fall of the Berlin Wall some 70 years later showed that man could not. He was not sufficiently motivated to live in that kind of economy. One thing you should understand: Russia lost 20 million people during WWII. The war fought in Russia was one of the worst wars ever fought anywhere. If you don't believe this, read about the siege of Leningrad or the military encounter at Stalingrad. Russia was greatly weakened by those two events.
The Great Depression looked like it weakened the American Economy. What really happened is that the World had to learn how to balance internal trade with external trade.
WWII brought America out of the Depression. The new Deal did not go far enough to get America out of her economic slump brought about the great depression, but WWII helped what the New Deal Started. Both worked hand in hand to bring about what America needed.
The Berlin Airlift cemented the trust people put in the United States believing that she was a moral victor who would help in times of trouble. People willingly became America's partner in trade.
Japan was treated humanely after the most inhumane event of WWII if not the 20th century. Instead of demanding reparations, America built up Japan. America knew that she would benefit from a strong Japan. The Marshall plan in Europe did the same thing.
Vietnam was an American policy that no one understood. Not deeply. I was in America at the time and even those who supported the war were uncomfortable participating in it. I met many young wives who were in tears many days during some part of the day wondering if their husbands would return. It was a horrible time. Worse economically for America, the people were told by LBJ that America was rich and powerful enough to afford guns and butter. It turned out to be partially true but with grotesque consequences. That's when the economy began to falter.
In the meantime China was beginning to rise. Napoleon predicted that when that happened, nothing would stop her. We've yet to see the end of that play out, but the two are in a wrestling match that is to the death of one of them.
By the end of the 20th century, America was strong both Militarily and economically, but both were beginning to falter. It is still B.
You could make an argument for E. I wouldn't do it. Not yet, but it is inevitable.
