Which of the following is an undesirable thermal property of a refrigerant?

The choice of low critical and high freezing temperatures as an undesirable thermal property of a refrigerant is correct because these characteristics can significantly affect the refrigerant's efficiency and operational range. A refrigerant with a high freezing temperature can solidify or crystallize under normal operating conditions, leading to blockages in the system. This solidification can restrict flow, impede heat exchange, and ultimately result in system failure or reduced performance.

Additionally, having a low critical temperature means that the refrigerant would not function effectively under higher pressure conditions, limiting its usability in many applications. This can lead to difficulties in maintaining proper refrigeration cycles, as the refrigerant may not vaporize or condense at the desired temperatures, causing inefficiency and potential operational issues.

In contrast, other properties such as high viscosity and low pour point, while potentially problematic in terms of flow and lubrication, do not directly impact the phase change efficiency as critically as the freezing and critical temperatures. As for high latent heat of evaporation and high film heat conductivity, these are generally desirable traits in refrigerants, as they contribute to effective heat transfer and energy efficiency when the refrigerant is used in a system.