Final answer:
Polyol ester oil is the most commonly used type of oil with HFC-134a refrigerant due to its compatibility and effective mixing with the refrigerant.
Step-by-step explanation:
The type of oil that is most commonly used in stationary refrigeration applications with HFC-134a refrigerant is Polyol ester oil (b). Polyol ester oils are synthetic lubricants used with HFC refrigerants. These oils are preferred due to their compatibility with the refrigerant and their ability to mix well over a wide range of temperatures. In contrast, mineral oils, which were used with older refrigerants like CFCs and HCFCs, are not typically used with HFC-134a because they do not mix well with the refrigerant. Innovations in environmentally friendly lubricants, including those derived from vegetable oils, show promise for reducing the environmental impact of lubricants.