Final answer:
option b.The most desirable property of ammonia refrigerant oil is being free of water to prevent ice formation and corrosion within the refrigeration system. High paraffin content and a low flash point are not considered desirable properties.
Step-by-step explanation:
A desirable property of ammonia refrigerant oil includes being free of water. This characteristic is paramount because water in the refrigerant oil could lead to ice formation, which can obstruct the refrigeration system's flow or cause corrosion.
High paraffin content is generally not desirable because it can lead to the formation of wax deposits that may clog the system. A very low flash point is also not a desired property for ammonia refrigerant oil as it signifies a higher risk of fire or explosion; thus, safety concerns make a higher flash point preferable.
Therefore, the correct answer is B) being free of water. It's important to have knowledge about properties like flammability, boiling and melting points, and solubility when working with chemicals such as ammonia to ensure both effective performance and safety.