Answer:
B. Member nations had to defend other members if they were attacked.
Step-by-step explanation:
The League of Nations was an international organization designed to preserve peace between those countries. President Woodrow Wilson was the principal architect of the League, the problem was that the Senate rejected the League in 1920. This was a part of a non-interventionist policy of the US during that time, the necessity of defending other members of the League if they were attacked was a major factor of this decision, the defeat was led by the Irreconciables, a bipartisan group of Senators who opposed the Treaty.