The correct answer is B) secure the salt water supply to the scrubber. When securing an inert gas system, the final step should be to secure the salt water supply to the scrubber. This is because the scrubber is responsible for removing any impurities from the inert gas before it is distributed throughout the system. Securing the salt water supply ensures that the scrubber can no longer function, effectively shutting down the entire inert gas system. The other options are incorrect because they do not completely shut down the inert gas system. Closing the flue gas isolating valve (A) or the deck isolating valve (C) would still allow the system to potentially generate or distribute inert gas, and securing the inert gas blower (D) would not prevent the system from drawing in air or other gases.