What were the events that have become known as 9/11?

On September 11, 2001, militants with the Islamic extremist group hijacked four airplanes and  s uicide attacked against targets in the United States. Two of the planes flown into the twin towers, a third plane hit the Pentagon just outside Washington, D.C., and the fourth plane crashed in a field in Shanksville, Pennsylvania.

Explanation:

d) post-world war i