According to 46 CFR Part 147, a cylinder used for storing CO2 in a fixed firefighting system must be hydrostatically retested and restamped every __________.

Correct Answer: Option D — 12 years

46 CFR 147.60 mandates that CO2 cylinders used in fixed firefighting systems must undergo hydrostatic testing and restamping every 12 years. This ensures the structural integrity of the pressure vessel remains capable of safely containing the high-pressure carbon dioxide charge required for effective emergency fire suppression.