The correct answer is B) through the purge unit vent. Explanation: Low-pressure refrigeration systems, such as those used on ships, often have a purge unit that vents small amounts of refrigerant to the atmosphere to remove non-condensable gases and maintain system efficiency. This is the primary way refrigerant is typically released during normal operation, as it is designed to vent the refrigerant in a controlled manner rather than through leaks or other unintended pathways. The other options are incorrect because: A) water-side leaks would be less common in a properly maintained system, C) compressor shaft seals are designed to minimize refrigerant release, and D) a rupture disk is an emergency safety device, not a normal release point.