Answer:
In my opinion it was the Global Economic Depression .
Step-by-step explanation:
Although all mentioned factors had a lot to do with the outbreak of WW2, I am choosing global economic depression because it led to creation of totalitarian regimes in the mentioned countries, including Germany and Italy.
These regimes used the economical crisis to accuse different ideologies for the outbreak of the depression. Therefore, I am choosing it. I would not choose Failure of The League of Nations and British and French Appeasement , as these events weren't as impactful as others that are mentioned in here.