If an oil separator is not used:

The choice indicating that oil traps should be installed in all vapor-carrying lines is correct because the primary function of an oil separator is to manage the oil circulating in a refrigeration system. Without an oil separator, it becomes essential to prevent oil from accumulating in the evaporator or other components that carry vapor. By installing oil traps in these vapor lines, technicians can ensure that oil is captured and returned to the compressor, maintaining proper lubrication throughout the system and ensuring efficient operation.

This approach prevents oil from obstructing the flow of refrigerant and enhances the overall reliability and efficiency of the refrigeration unit. Proper oil management is crucial in avoiding potential system failures, increases in energy consumption, and inefficiencies in cooling performance.