The correct answer is B) The door must be kept unlocked. According to 46 CFR Part 95, the regulations for the carbon dioxide cylinder room of a CO2 fixed fire extinguishing system state that the door must be kept locked to prevent unauthorized entry. This is to ensure the system remains secure and can be accessed only by authorized personnel in the event of an emergency. The other statements are all true - the ambient temperature must not exceed 130°F, the compartment must be properly ventilated, and it must be clearly identified and marked.