When using a recovery unit on a system containing 100 lbs. of R-410A it must be evacuated to?

Evacuating a refrigerant system to 0 inches of mercury means that the system is being brought to a complete vacuum. This is essential when recovering refrigerants like R-410A, as it ensures that all refrigerant is removed from the system, allowing for safe servicing or disposal.

Achieving a full vacuum eliminates any remaining refrigerant and contaminants, which is vital for preventing harm to the environment and ensuring compliance with regulations.

In the context of refrigerant recovery, achieving lower gauges like 0 inches of mercury indicates that there's no pressure left in the system, thereby confirming that all R-410A has been adequately recovered and there are no potential leaks in the system. The other options represent partial vacuums, which would not be sufficient for complete recovery and would not adhere to best practices outlined in EPA regulations and standards for refrigerant handling.