Answer:
Secret.
Explanation:
Derivative classification involves the incorporation of new information which has previously been classified and the labeling of the new document as per its classification. It identifies the authorized sources while classifying with the concepts 'revealed by' and 'contained in'.
As per the question, the statement in the new document could be classified as secret as it exemplifies confidential information relating to the length of the training exercise. Thus, it could be classified as 'secret'.