The correct answer is A) fusible link. In the event of an exhaust duct fire, most dry chemical and carbon dioxide galley range fixed extinguishing systems are automatically activated through the action of a stainless steel cable, spring, and a fusible link. The fusible link is designed to melt at a specific temperature, typically around 165-212 degrees Fahrenheit, triggering the release of the extinguishing agent and suppressing the fire. This automatic activation mechanism is a critical safety feature required by regulations for commercial cooking equipment to prevent the spread of galley fires. The other options are incorrect because: B) a stack switch is used to detect changes in airflow, not temperature; C) a thermostat measures and controls temperature, but does not automatically trigger the extinguishing system; and D) a pyrostat is a type of heat-sensitive device, but is not the specific mechanism used in these galley range fire suppression systems.