Final answer:
Republican senators objected the most to the requirement of the United States to send soldiers at the request of the League of Nations in the Treaty of Versailles.
Step-by-step explanation:
The part of the Treaty of Versailles that Republican senators objected to the most was D.) Requiring the United States to send soldiers at the request of the League of Nations. They were concerned that joining the League of Nations would compromise U.S. sovereignty and draw the nation into foreign disputes without the Senate's constitutional power to declare war. The fear of losing control over U.S. military involvement and the objection to Britain and France's influence over the League were the main reasons for the Senate's opposition to this provision.