Why did congress reject the treaty of versailles

Because of the objections to the League of Nations. The U.S. would never ratify the treaty or join the League of Nations.

 The Senate worried about the US being pulled into future wars that were not directly related to US national security.

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.

d is the right answer

to replace ineffective because   andrew jackson's spoil system was a deliberate policy after he became president to remove federal employees he considered to be political opponents and replace them with his own supporters.