Which of the following refrigerants was phased out due to its impact on ozone depletion?

R-12, also known as dichlorodifluoromethane, was a commonly used refrigerant in air conditioning and refrigeration systems. It is classified as a chlorofluorocarbon (CFC), which are compounds known for their negative impact on the ozone layer. Research indicated that when CFCs are released into the atmosphere, they can undergo photodissociation, releasing chlorine atoms that can deplete ozone molecules. As a result of this environmental concern, the use of R-12 has been severely restricted and phased out under the Montreal Protocol, an international treaty designed to protect the ozone layer by eliminating substances responsible for ozone depletion.

In contrast, R-134a and R-404A are hydrofluorocarbons (HFCs) that do not contain chlorine and thus do not contribute to ozone depletion. R-152A is also a hydrocarbon with a low ozone depletion potential. Therefore, R-12 is the clear choice when considering refrigerants that were phased out explicitly for their harmful effects on the ozone layer.