The most accurate summary of the United States' involvement and military role in World War I from the options provided is:
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 proposing a military alliance against the U.S., played a crucial role in prompting U.S. entry into the war, along with other factors such as unrestricted submarine warfare by Germany.