Which of the following refrigerants is classified as a very high-pressure refrigerant?

R-744, commonly known as carbon dioxide (CO2), is classified as a very high-pressure refrigerant. In refrigeration systems that utilize R-744, the operating pressures can be significantly higher compared to traditional refrigerants. This characteristic is particularly pronounced in transcritical cycles where the refrigerant operates above its critical point, leading to high pressure in both the condensing and evaporating phases.

The other refrigerants listed, while having their own operational pressures, do not reach the same magnitude of high pressure as R-744. CFC-11 (a chlorofluorocarbon) and HCFC-123 (a hydrochlorofluorocarbon) operate under much lower pressure conditions, typical of halocarbon refrigerants, which are designed for different applications. R-410A, though also a high-pressure refrigerant relative to others in its category, operates at pressures that are lower than those encountered with R-744 in its intended applications, especially in critical cycles. Therefore, R-744 is indeed notable for its very high-pressure classification in comparison to the others listed.