It would be "Indonesia" that is not a country of mainland Southeast Asia," since Indonesia is in fact a large groups of islands off the coast, located above Australia.
The purpose of the League of Nations was to prevent through collective security and disarmament and settling international disputes through negotiation and arbitration. The League of Nations also wanted to take care of labor conditions, just treatment of native inhabitants, human and drug trafficking, arms trade, global health, prisoners of war and protection of minorities in Europe.
The question will be true! :D
A,B,C. If this is incorrect then my bad.