Final answer:
The correct banner marking for a document containing TOP SECRET, SECRET, and CUI is TOP SECRET, as documents are marked with the highest classification level present.
Step-by-step explanation:
The correct banner marking for a co-mingled document containing TOP SECRET, SECRET, and CUI (Controlled Unclassified Information) is TOP SECRET. In the classification hierarchy, documents are marked with the highest level of classification present among the information included. Therefore, if a document contains TOP SECRET information, the entire document must be handled as such, regardless of the presence of lower classification levels or CUI within the same document.