The correct answer is C) weight. The amount of HCFC-123 in a storage cylinder is measured by weight because HCFC-123 is a liquid refrigerant, and the weight of the liquid provides the most accurate measure of the quantity contained in the cylinder. The volume or saturation pressure/temperature can be affected by factors like temperature and cylinder size, so weight is the most reliable and commonly used method to determine the HCFC-123 content. The other answer choices are incorrect because they do not provide a direct measurement of the refrigerant quantity. Volume can vary, saturation pressure and temperature are indirect indicators that can change based on conditions, but the weight of the liquid refrigerant is the definitive way to quantify the HCFC-123 content in the storage cylinder.