The correct answer is D) calibrate the gas leak detectors. Span gas is a known concentration of a specific gas used to calibrate and verify the accuracy of gas detection equipment aboard liquefied natural gas (LNG) carriers. These gas detectors are critical safety devices that monitor for potential leaks in the cargo piping and storage tanks. Calibrating the detectors with span gas ensures they can accurately identify and quantify any release of LNG vapors, which is essential for the safe operation of these specialized vessels. The other answer choices are incorrect because: A) odorizing the cargo is not required for LNG, which is an odorless gas; B) detecting leaks is the purpose of the gas detectors, not the span gas itself; and C) inerting the barrier spaces is a separate safety system, not the function of the span gas.