Which group is considered second in priority for using the flight deck observer seat?

The correct choice for the second priority group for using the flight deck observer seat is Air Canada evaluators. This group is specifically tasked with monitoring and assessing flight operations to maintain safety and efficiency. Their role often includes conducting evaluations of pilots and ensuring compliance with industry regulations and company policies.

Transport Canada, mentioned in another choice, holds regulatory oversight but does not directly use the seat in a priority context in the same way that evaluators from Air Canada do. Holders of authorization letters and operating relief pilots would also have different levels of priority based on operational needs and official protocols in place. Thus, the position of Air Canada evaluators as the second group highlights its importance in overseeing and improving flight operations.