Answer: a. US membership in the League of Nations
Step-by-step explanation:
After the first World War, President Wilson wanted world nations to set up a League of Nations that would monitor and ensure peace in the world. Senator Lodge was very much against joining this organization as he believed that it would erode US sovereignty and unnecessarily yoke them together with Europe and its many troubles.
He therefore campaigned against the US joining in the Senate and with his influence, he was successful which meant the US mostly stayed out of European politics until the second world war.