What was the name of the secret unit established that remained undisclosed for 35 years?

The secret unit that remained undisclosed for 35 years is correctly identified as the Secret Investigative Unit. This name suggests a focus on covert operations and sensitive investigations, aligning with the nature of its undisclosed activities. The longevity of its secrecy indicates that it likely dealt with high-stakes situations or impactful investigations that required confidentiality, a characteristic typical of units engaged in sensitive criminal intelligence work.

The other choices, while plausible names for various investigative entities, do not carry the same implications of prolonged secrecy. The titles Special Operations Team, Criminal Intelligence Committee, and Undercover Division could refer to different types of units that might not emphasize the clandestine aspect inherent in the term "Secret Investigative Unit." The lack of public knowledge about this unit for 35 years further underscores its unique function and necessity for discretion in its operations.