Final answer:
For leak checking an HVAC system after recovering R-410A refrigerant, nitrogen should be used due to its inert and non-flammable properties, ensuring safe and effective testing.
Step-by-step explanation:
After proper R-410A refrigerant recovery, you should use nitrogen to leak check the system. Options such as oxygen or flammable gases are hazardous because they can create a potential fire hazard when combined with oil and the heat of operation. Nitrogen is an inert, non-flammable gas that is commonly used for pressure-testing and leak-checking in HVAC systems. Not only does it not react with the system's components, but it also allows for the testing of system integrity without the risk of a refrigerant release, which can be both environmentally damaging and costly.