Which of the following is a major refrigerant that has been phased out due to environmental concerns?

The correct choice is B, CFC-12, because this refrigerant is classified as a chlorofluorocarbon (CFC), which has significant environmental impacts, particularly in relation to ozone depletion. CFC-12 was widely used in refrigeration and air conditioning systems until its damaging effects on the ozone layer became widely recognized. The international community responded to these concerns with measures such as the Montreal Protocol, which outlined the phase-out of CFCs and other ozone-depleting substances.

In contrast, HFC-134a is a hydrofluorocarbon that, while still facing regulatory scrutiny due to its global warming potential, was introduced as a replacement for CFCs and does not deplete the ozone layer. R-410A is a newer refrigerant mixed with HFCs designed to have minimal ozone impact, and R-22, also phased down due to its past CFC classification and high ozone-depletion potential, is still used in some applications but is subject to strict phase-out regulations. However, CFC-12 is particularly noteworthy for its direct association with ozone layer depletion, leading to its definitive phase-out.