What technical control provides the most critical layer of defense if an intruder is able to bypass all physical security controls and obtain tapes containing critical data?

Camera recordings are a detective control that may document the breach after the fact but provide no protection for the data already in the attacker's hands.

Security guards are a physical preventive control that the scenario explicitly states have already been bypassed, so they offer no protection at this stage.

Encryption protects data at rest by making the contents of stolen tapes computationally infeasible to read without the corresponding decryption key - even with full physical access to the media, the attacker cannot recover plaintext data, directly preserving confidentiality after all physical controls have failed.

Shredding is a data destruction method applied at end-of-life for media, not a control applicable to tapes that are still in active use and have already been stolen.

Corrective controls restore systems and operations after an incident but do not protect the confidentiality of data that an attacker already possesses.