Each ship having an inert gas system must have a portable instrument to measure concentrations of hydrocarbon vapor in inert atmospheres and also to measure __________.

Correct Answer: Option A — oxygen

Inert gas systems must maintain oxygen levels below 5% to prevent combustion within cargo tanks. Portable oxygen analyzers are mandatory for verifying the inert atmosphere, ensuring that the oxygen concentration remains below the threshold required to support ignition, as required by SOLAS and USCG safety regulations.