Final answer:
All the listed safety precautions should always be followed when handling refrigerants and compressed gases to prevent dangerous incidents such as chemical reactions, explosions, and toxic exposure.
Step-by-step explanation:
When dealing with refrigerants and compressed gases, safety is of utmost importance due to the potential hazards they present. The safety precautions that should always be followed include: never applying an open flame to a refrigerant cylinder, never cutting or welding any refrigerant line when refrigerant is in the line, and never using oxygen to purge lines or to pressurize a system. Therefore, the correct answer is All of the above. It's important to understand that these precautions are crucial not only for the safety of the individual handling the gases but also for the safety of others and the environment. Failure to adhere to these precautions could result in dangerous chemical reactions, explosions, and toxic exposure.