The answer should be B. through both diplomacy and strength of arms
This was a quote after WWI, the "war to end all wars". At the end of the war, Germany was basically blamed for the war in the Treaty of Versailles. They had to pay an immense amount of money to the other countries. In WWII, Germany got its "revenge".
The Union victory did solve a handful of problems, it abolished slavery and get rid of the Confederate Union but not every problem could be solved. Personally I would disagree because the Civil War had other problems right after because of the victory.
