The correct answer is A) supply and exhaust ventilation. When the CO2 fixed fire extinguishing system is activated, it causes the shutdown of both the supply and exhaust ventilation systems. This is a critical safety measure to prevent the CO2 from being vented out and to maintain the high concentration of CO2 within the protected space to effectively extinguish the fire. The other options are incorrect because: B) shutting down only the exhaust ventilation would not be sufficient to contain the CO2; C) shutting down the fuel supply is a separate action, not directly related to the CO2 system activation; and D) shutting down both mechanical and natural ventilation is too broad, as the key action is the shutdown of the supply and exhaust ventilation specifically.