Which of the following was a reason that the us senate refused to let the united states join the league of nations?

You didn't provide choices, but the main reason for refusing entry into the League of Nations was the belief that doing so meant giving up some of the United States' own sovereignty and could commit the US to defend other nations' security rather than its own.

Context/detail:

The United States never joined the League of Nations, in spite of the fact that an organization such as the League of Nations was the signature idea of US President Woodrow Wilson.  He had laid out 14 Points for establishing and maintaining world peace following the Great War (World War I).  Point #14 was the establishment of an international peacekeeping association. The Treaty of Versailles adopted that idea, but back home in the United States, there was not support for involving America in any association that could diminish US sovereignty over its own affairs or involve the US again in wars beyond those pertinent to the United States' own national security.   Because of its objections to membership in the League of Nations, the United States Senate refused to ratify the Treaty of Versailles.

the emancipation proclamation was the aim of the war change to include the freeing of slaves in addition to preserving the union. the 13th amendment, which abolished slavery in the u.s. was passed on december 6th 1865.

as promised, lincoln waited to unveil the proclamation until he could do so with success from the union military advance. on september 22nd 1862, after a victory at antietam, he publicly announced a preliminary emancipation proclamation, declaring all slaves free in the rebellious states as of january 1st 1863.

the passage of a strict fugitive slave law