Select the correct NATO portion marking for NATO CONFIDENTIAL information in a U.S. document.

The correct marking for NATO CONFIDENTIAL information in a U.S. document is indeed marked as (//NATO NC). This designation is important because it aligns with the NATO classification levels, which categorize information based on its sensitivity and the potential impact of unauthorized disclosure.

NATO CONFIDENTIAL, as a classification level, indicates that the information, if disclosed, could harm NATO's interests but is not deemed as severely damaging as higher classification levels like NATO SECRET or NATO TOP SECRET. The specific abbreviation 'NC' directly corresponds to the NATO classification structure, ensuring that personnel who handle such information recognize its sensitivity and handle it appropriately.

Proper marking is vital for maintaining the security of classified information, as well as for ensuring compliance with both NATO and U.S. regulations on handling sensitive material. Understanding and correctly applying these markings help to protect national security interests and foster trust among allied nations.