The most important cause of WW2 (as of WW1) was imperialism. ... Japan started the war due to imperialist motives – to conquer China and Oceania for their resources and manpower. Neither the treaty of Versailles nor the failure of the League of Nations are causes for the Second Word war.
In my own opinion I think that the answer is D because Germany was the main reason why we had WW1 and WW2.
I hope my answer helps!!
:)