What must be placed on a refrigerant cylinder being returned for reclaiming?

When returning a refrigerant cylinder for reclaiming, it is essential to place a refrigerant label on the cylinder. This label serves critical functions, including indicating the type of refrigerant contained within the cylinder, ensuring proper handling and processing during the reclaiming process. It provides vital information to facilities that handle refrigerants by ensuring they can identify and process the refrigerant correctly, adhering to environmental regulations and safety standards.

Using a refrigerant label helps prevent contamination and mismanagement of different refrigerant types, which can be harmful to the environment and human health if improperly handled. This is why the presence of the refrigerant label is a requirement before the cylinder is returned for reclaiming. The other options, while potentially relevant in different contexts, do not fulfill the specific regulatory and practical requirements necessary for returning a refrigerant cylinder.