Catalysts for U.S. Entry into World War I

As World War I escalated, the United States faced critical decisions that would shape its engagement in the conflict. By 1917, various factors pushed the U.S. towards intervention, including economic ties, unrestricted submarine warfare by Germany, and events like the Zimmermann Telegram. These elements significantly impacted public opinion and government decisions regarding involvement in the war. Which of the following events or developments was the most direct catalyst for the U.S. entry into World War I?