Which of the following is a reason why the United Nations had a more successful start than the League of Nations?

I don't see a list, but part of the reason is the rejection of the League of Nations by the United States, rendering it essentially meaningless as an organization. There was the general sense in America that we had sacrificed U. S. lives rescuing the Allies (Britain and France) and received no recompense, even a just peace. This prompted the Isolationist movement which determined to disengage from European conflicts as being of no import to a virtually economically independent United States. Without the primary western nation as a member, when the League of Nations was finally confronted with instances of international aggression (such as the Japanese invasion of coastal North China, or the Italian invasion of Ethiopia) it found itself powerless to intervene, thus rendering itself utterly meaningless.

The United Nations came into being in the wake of a much more destructive war (World War Two), which was concluded in part because of the deployment of new weapons (atomic bombs) that contained unprecedented power which could be deployed very quickly, virtually without warning. With these new more dangerous conditions it made sense to provide a venue where all the main nations of the world could communicate and discuss potential conflicts before they developed into actual conflict. Additionally it was recognized that a "buffer" force made up of independent military from many nations would be preferable to act as peacekeepers in areas that might otherwise quickly develop into combat if potential opponents had to face one another directly.