The correct answer is A) the space requiring the largest amount. The reasoning behind this is that the fixed carbon dioxide extinguishing system aboard a cargo vessel must be sized to provide sufficient CO2 to extinguish a fire in the single largest space on the vessel, which is typically the largest cargo hold. The system does not need to be sized for all cargo spaces or the entire vessel, as the CO2 supply only needs to be sufficient for the space with the greatest firefighting requirements. The other options are incorrect as they either require too much or too little CO2 coverage for the vessel.