Question 1139 Deck General

The correct answer is B) oxygen. Vessels with inert gas systems must have a portable instrument to measure not only the concentration of hydrocarbon vapor in the inert atmosphere, but also the oxygen content. This is a critical safety requirement, as maintaining the proper oxygen level in the inert gas is essential to prevent the risk of explosions or fires on board. The other options are incorrect because nitrogen (A) and carbon dioxide (C) are not the key atmospheric components that need to be monitored, and water vapor (D) is not as critical to measure as oxygen levels in this context.