Which of the following is NOT considered a banner marking?

A banner marking is a specific label used to indicate the classification level of information or material. These markings provide immediate visibility regarding the sensitivity of the information contained within a document and help ensure proper handling and safeguarding.

The option indicating "Classified Open" does not correspond to any recognized classification level. The standard classification levels recognized are TOP SECRET, SECRET, and CONFIDENTIAL, each of which has distinct handling procedures and implications regarding access and distribution. Since "Classified Open" does not adhere to the established classification schema and lacks clarity as a classification category, it is not considered a banner marking.

In contrast, the other options—TOP SECRET, CONFIDENTIAL//NOFORN, and SECRET—are indeed standard banner markings clearly identifying the sensitivity of the information, making it crucial for users to recognize and treat the information according to its corresponding classification.