What is the highest classification permitted for performance report supplements when classified comments are necessary?

The highest classification permitted for performance report supplements when classified comments are necessary is Secret. This classification level allows for the inclusion of sensitive information that could cause serious damage to national security if disclosed. Performance reports often contain evaluations and assessments that may require the safeguarding of specific details, thus necessitating a classification level that ensures protection while still providing necessary information to authorized personnel.

Confidential does not offer the same level of protection required for potentially damaging information, while Top Secret relates to information that could cause exceptionally grave damage if revealed—this is too high for the context of performance reporting where a Secret classification suffices. Restricted is not a classification standard used for handling classified information in this context, further highlighting why Secret is the correct answer.