Final answer:
Refillable refrigerant cylinders should never be filled beyond 80% capacity to ensure safety and account for gas expansion.
Step-by-step explanation:
The subject question asks about safe practices for filling refillable refrigerant cylinders. The correct answer to the question, "Never fill a refillable refrigerant cylinder beyond ________." is a) 80% capacity. Refrigerant cylinders should not be filled beyond this limit to allow for expansion of the refrigerant gas and to ensure safe handling and storage.