Final answer:
None of the listed refrigerants (R11, R12, R134a, R22) can be mixed as it could harm the system and is generally against regulations. Separately, Freon-12 is a gas at room temperature as its boiling point is below 25°C.
Step-by-step explanation:
The question addresses the compatibility of refrigerants for mixing purposes. The correct answer to the question "Which refrigerant can be mixed?" is D) none of the above. Refrigerants should not be mixed because each one has been designed with specific properties that could be compromised when mixed with other substances. Mixing different refrigerants can lead to issues such as reduced cooling efficiency, increased wear on the system, potential chemical reactions, and it could violate regulations related to refrigerant use.
As for Freon-12 (also known as chlorofluorocarbon-12 or CFC-12), based on its melting point of −157.7°C and boiling point of −29.8°C, Freon-12 would be in a gaseous state at room temperature (25°C) since this temperature is above its boiling point.