The answer is option 4: Membership in the League could draw the United States into future wars.
Being part of the League of Nations meant being part of an intergovernmental organization aiming to prevent future wars by uniting nations that would not fight each other. It intended to achieve this through collective security as well as disarmament and by settling international disputes through negotiation and arbitration, among others.
However, it also meant that if any outside country ever attacked any member of the League of Nations, the other member nations would defend, protect and fight on behalf of the attacked country. The American senators were aware of this, and after the devastation of the World War I (1914-18), most Americans were sure they didn't want to involve in any foreign affairs that weren't aligned with their domestic interests and decided to return to their Isolationist policy instead.