What major event occurred on September 11, 2001, in the United States?

The major event that occurred on September 11, 2001, in the United States, was a coordinated terrorist attack carried out by the extremist group al-Qaeda. Members of this organization hijacked four commercial airplanes, crashing two into the Twin Towers of the World Trade Center in New York City and a third into the Pentagon in Arlington, Virginia. The fourth plane, United Airlines Flight 93, was also hijacked but ultimately crashed into a field in Pennsylvania after passengers attempted to overcome the hijackers. This tragic event resulted in significant loss of life, profound changes to U.S. domestic and foreign policy, and heightened security measures nationwide.

This answer is correct as it directly identifies and describes the nature and outcome of the events of 9/11, accurately reflecting the historical impact of the attacks. The other options do not relate to the events of that specific day; for example, a natural disaster would imply a catastrophic event caused by nature, a military coup involves the overthrow of a government, and a presidential assassination pertains to the unlawful killing of a sitting president, none of which occurred on that date.