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

R-12 is a common refrigerant that was widely used in air conditioning and refrigeration systems. It has been phased out due to significant environmental concerns, particularly its impact on the ozone layer. As a chlorofluorocarbon (CFC), R-12 contributes to ozone depletion when released into the atmosphere. The Montreal Protocol, an international treaty aimed at protecting the ozone layer by phasing out the production and consumption of ozone-depleting substances, led to the discontinuation of R-12 in favor of alternatives that have less harmful effects on the environment.

In comparison, R-134a and R-410A are more recent refrigerants that do not contain chlorine and, therefore, do not deplete the ozone layer, although R-134a has been scrutinized for its global warming potential. R-22, while also phased down under environmental regulations, has not been entirely banned yet but is being replaced by more eco-friendly options. R-12's significant role in ozone depletion and its complete phase-out under regulatory measures make it the correct answer for this question.