The correct answer is B) left closed with ventilation off until all boundaries are cool. The reasoning for this is that after extinguishing a fire in a paint locker using the fixed CO2 system, the space should be left closed to maintain the reduced oxygen environment and prevent reignition. Leaving the space closed prevents the introduction of fresh air that could cause the fire to reflash. The boundaries of the space, including bulkheads and overhead, need to be thoroughly cooled before the space is reopened to ensure the fire is completely extinguished. The other options are incorrect because: A) immediately opening the space and removing burned material could introduce oxygen and cause the fire to restart; C) checking the oxygen level is not the immediate next step after extinguishing the fire; and D) dousing the space with water is not necessary if the CO2 system has effectively extinguished the fire.