Which attack method has NOT been attempted against an airport or airline since September 11, 2001?

The correct answer is the method involving chemical or biological attacks. Since September 11, 2001, there have been numerous incidents targeting airports and airlines that involved bombings, hijackings, and mass shootings. These methods have been attempted both in the U.S. and around the world, indicating that they remain a concern for aviation security.

In contrast, there have not been documented attempts involving chemical or biological attacks specifically targeting airports or airline operations after this date. This absence underscores a shift in the nature of threats faced by aviation security, where attackers have gravitated towards more immediate and visible forms of violence, such as bombings and shootings, rather than relying on more complex and logistical approaches typical of chemical or biological weapons, which may be harder to carry out effectively in such environments. Therefore, the statement regarding chemical or biological attacks being an unattempted method since 2001 is supported by the lack of recorded incidents in that specific category.