The correct answer is A) secure all sources of fresh air to the tank. The reasoning behind this is that in the event of a fire in a fuel tank, the most important first step is to eliminate the source of oxygen that is feeding the fire. By securing all sources of fresh air to the tank, such as ventilation systems or openings, the fire will be starved of the oxygen it needs to continue burning. This is a critical first action that must be taken before attempting other firefighting measures. The other answer choices are incorrect because: B) stationing someone at the fixed CO2 release controls is a secondary action, not the first step; C) topping off the tank would actually increase the fuel vapors, making the situation more dangerous; and D) transferring the fuel to other tanks should only be attempted if the fire is contained and under control.