Final answer:
The largest terrorist attack in U.S. history occurred on September 11, 2001, when 19 Al-Qaeda terrorists hijacked four planes, leading to nearly 3,000 deaths and the beginning of the War on Terror.
Step-by-step explanation:
The date that marks the largest terrorist attack in U.S. history is c) September 11, 2001. On this day, 19 terrorists affiliated with the extremist group Al-Qaeda, led by Osama bin Laden, hijacked four commercial airliners. Two planes were flown into the Twin Towers of the World Trade Center in New York City, one crashed into the Pentagon in Washington, D.C., and the last plane, United Flight 93, crashed into a field in Pennsylvania after passengers attempted to retake control from the hijackers. This coordinated attack resulted in the deaths of nearly 3,000 people, including approximately 400 emergency responders, and precipitated major global events, including the initiation of the War on Terror.
Prior to the September 11 attacks, the deadliest terrorist attack on American soil was the Oklahoma City bombing on April 19, 1995. However, the scale and impact of 9/11 were significantly greater, marking it as an unprecedented terrorist event in U.S. history.