Which substance is least damaging to the stratospheric ozone layer?

The chosen answer is correct because Hydrochlorofluorocarbons (HCFCs), while still having some ozone depletion potential, are significantly less damaging to the stratospheric ozone layer compared to the other substances listed. HCFCs were developed as transitional substitutes for chlorofluorocarbons (CFCs) and halons, which are known to cause substantial damage to the ozone layer.

CFCs and halons are highly effective at depleting ozone due to their stable molecular structure that allows them to travel up into the stratosphere, where they are broken down by UV radiation, releasing chlorine and bromine atoms that can destroy ozone molecules. Methyl bromide is similarly harmful, as it also contributes to ozone depletion.

In contrast, HCFCs contain hydrogen, which makes them less stable and allows them to break down more quickly in the lower atmosphere, thus reducing their potential to reach the stratosphere and cause ozone depletion. This characteristic of HCFCs, along with regulatory measures to phase them out in favor of even less harmful alternatives, underscores why they are considered the least damaging among these options.