The correct answer is C) 90%. According to the regulations, CO2 cylinders must be recharged when the weight of the charge in the cylinder is less than 90% of the stamped full weight of the charge. This is to ensure that the cylinder always contains the minimum required amount of CO2 for its intended use, such as fire extinguishing or other safety purposes. The other answer choices are incorrect because 80% (A) and 85% (B) are below the 90% threshold, while 95% (D) is above the required minimum.