What is the minimum ambient temperature that heat transfer liquids must be protected from freezing?

The correct choice indicates that the minimum ambient temperature is the lowest temperature at which heat transfer liquids must be protected from freezing. This is key for ensuring the efficiency and safety of heating systems that utilize these liquids. If these heat transfer liquids freeze, it can lead to system failure, damage to pipes, or the equipment relying on these fluids.

Thus, specifying the lowest ambient temperature ensures that adequate measures, such as the use of antifreeze solutions or insulation, are taken to prevent freezing and ensure the continuous operation of the system even in the coldest conditions likely to be encountered. Understanding the lowest temperature allows engineers and technicians to design systems that maintain functionality and avoid costly repairs or safety hazards.