Which of the following refrigerants is considered an ozone-depleting substance?

R-22 is considered an ozone-depleting substance because it contains chlorine, which has been shown to contribute to the depletion of the ozone layer when it is released into the atmosphere. The chlorine atoms can break down ozone molecules, leading to a thinning of the ozone layer that protects the Earth from harmful ultraviolet radiation.

The other options, including R-410A, R-134a, and R-404A, do not contain chlorine and are not classified as ozone-depleting substances. R-410A is a blend of two hydrofluorocarbons (HFCs) that have no ozone-depleting potential. R-134a and R-404A are also HFCs and are recognized for their low impact on the ozone layer. Therefore, R-22 is the only refrigerant among the choices listed that is associated with ozone depletion due to its chlorine content.