Before shipping any used refrigerant cylinders, check that the cylinder meets what standards:

The correct choice is related to the requirement that any used refrigerant cylinders must be compliant with the Department of Transportation (DOT) standards. The DOT regulates the transportation of hazardous materials, including refrigerants, ensuring that cylinders are manufactured, tested, and maintained according to safety standards. This is crucial for preventing leaks, ruptures, or other failures during transport, which could result in environmental harm or safety hazards.

While the other options mentioned have specific importance in the context of refrigerants, they do not directly pertain to the transportation of used refrigerant cylinders. The Significant New Alternatives Policy (SNAP) involves evaluating and regulating alternatives to ozone-depleting substances, while Underwriters Laboratories (UL) focuses on product safety certifications. Thus, while both are pertinent in the broader context of refrigerant handling and environmental safety, it is the DOT standards that specifically govern the transportation aspect of used refrigerant cylinders.