What can be done with refrigerant recovered from an air-conditioning system that is held in a refillable cylinder for coil replacement?

The selected answer is correct because refrigerant that has been recovered from an air-conditioning system and is held in a refillable cylinder can be charged back into the same system, provided that it is still in compliance with the applicable regulations and is free of contaminants. This is a common practice in HVAC maintenance and repair, as it allows for the reuse of the refrigerant, minimizing waste and saving costs.

When refrigerant is recovered, it is typically done in a way that it is captured in a clean and sealed cylinder. If it is determined to be in good condition and meets the system's specifications, it presents an efficient solution to refill the system without needing to source new refrigerant. This approach aligns with environmental stewardship practices, as it encourages the recycling and responsible management of refrigerants, which are regulated substances due to their potential impact on the ozone layer and global warming.

The other options involve actions that may not be necessary or permissible without further evaluation of the refrigerant's condition.