Which of the following is NOT required to be returned to the department upon cancellation of an inspection station?

The correct choice, which indicates what is not required to be returned to the department upon cancellation of an inspection station, is the driver's license. The driver's license is a personal identification document held by individuals and does not belong to the inspection station itself or relate to its operations.

In contrast, unused certificates of inspection, mechanic certification cards, and signature cards are all documents directly associated with the functioning of an inspection station. Unused certificates of inspection must be returned as they represent potential inspection approvals that could falsely be issued. Mechanic certification cards are critical for verifying that the personnel performing inspections have met the necessary training and certification requirements, and thus must be returned when an inspection station is closed down. Signature cards are also essential for maintaining accurate records of individuals authorized to sign inspection certificates and should be relinquished upon the station's cancellation to ensure proper oversight.

Understanding the distinctions between personal identification documents, like a driver's license, and operational documents related to the inspection station helps clarify why the driver's license does not need to be returned.