The most accurate summary of the United States' involvement and military role in World War I is:
A. The U.S. became involved in World War I after the Zimmermann Telegram, and sent troops to provide military reinforcements and support.
The Zimmermann Telegram, sent by Germany to Mexico, proposed a military alliance against the U.S., which was a significant factor in prompting U.S. entry into the war. When the U.S. joined the conflict in April 1917, it sent troops to provide support to the Allied forces.