Final answer:
One key lesson learned from 9/11 regarding anti-hijacking strategy is the importance of reinforcing cockpit doors on airplanes.
Step-by-step explanation:
A key lesson learned from 9/11 regarding anti-hijacking strategy is the importance of reinforcing cockpit doors on airplanes. Retrofitting all U.S. planes with reinforced cockpit doors has made it much harder for terrorists to take over the plane and gain control of the aircraft. This measure has significantly increased the safety and security of air travel.