What effect did World War I have on the United States?

The wartime economy kindled an increase in racial unrest across the country.

The country's credibility wavered because of the war's outcome in Europe.

The nation became a world leader as a member of the League of Nations.

The country experienced an economic recession due to wartime production.

has to be one of the answers above

The country experienced an economic recession due to wartime production.

A better question would have been why was the USA not part of the League of Nations despite the fact that a US president started it? Sometimes we really blow it.

You are correct that the United States was not part of the League of Nations, despite President Woodrow Wilson's role in its creation. The main reason for the U.S. not joining the League of Nations was due to opposition in the U.S. Senate. Many senators were concerned about Article X of the League's Covenant, which they believed could potentially lead the U.S. into unwanted military commitments without the approval of Congress. The Senate ultimately rejected the Treaty of Versailles, which included U.S. membership in the League of Nations, and the United States never officially joined.