Final answer:
The discovery of the Zimmermann Telegram was the event that caused President Wilson to ask Congress for a declaration of war against Germany. This secret communication proposed a German-Mexican alliance against the United States, which led to public outrage and influenced Wilson's decision to enter World War I.
Step-by-step explanation:
The event that finally caused President Woodrow Wilson to ask Congress for a declaration of war against Germany was c. the discovery of the Zimmermann Telegram (also known as the Zimmermann Note). While there were multiple factors that contributed to the United States' entry into World War I, it was the interception and decoding of the Zimmermann Telegram that pushed Wilson to take action. In this telegram, German Foreign Secretary Arthur Zimmermann proposed a military alliance between Germany and Mexico in the event that the United States entered the war against Germany. Zimmermann promised that Mexico would regain lost territories in Texas, New Mexico, and Arizona after the war. The British intercepted and decoded this message and shared it with the United States, causing public outrage and tipping the scales toward war.
Other events, such as the sinking of ships carrying Americans, also played a crucial role in shaping public opinion against Germany and pushing Wilson towards war. However, the Zimmermann Telegram was the more immediate cause for his decision. He presented the intercepted note to Congress, and on April 2, 1917, Wilson asked for a declaration of war to "make the world safe for democracy". The U.S. Congress declared war on Germany on April 6, 1917.