What should be the correct banner marking for a U.S. document that contains NATO CONFIDENTIAL information?

The correct banner marking for a U.S. document that contains NATO CONFIDENTIAL information is "CONFIDENTIAL//FGI NATO." This marking indicates that the information is classified at the Confidential level according to U.S. classification standards and also contains NATO information categorized under the NATO classification system.

The designation "FGI" stands for "Foreign Government Information," which is crucial in this context, as it shows that the document not only includes NATO-restricted content but also derives from foreign government sources, necessitating the specific handling and protection measures that FGI entails. This banner marking properly aligns with the requirement to mark documents containing NATO classified information, ensuring that individuals who access the information are aware of both the level of classification and the associated international considerations.

For the other choices, while they include elements relevant to classified markings, they do not correctly represent the classification level or the necessary components associated with NATO information, such as FGI, thus making them inappropriate for documents containing NATO CONFIDENTIAL content.