What does the term "substitute" refer to in the context of refrigerants?

In the context of refrigerants, the term "substitute" specifically refers to the replacement of ozone-depleting substances with alternative options that have a reduced impact on the ozone layer. This is crucial due to international agreements like the Montreal Protocol, which aim to phase out substances that harm the ozone layer.

Substitutes in refrigerants are designed to provide effective cooling without contributing to ozone depletion, ensuring that the HVAC systems are more environmentally friendly. This transition has led to the development of refrigerants like HFCs (hydrofluorocarbons), which, while still under scrutiny for their global warming potential, do not deplete the ozone layer.

The other choices focus on different aspects of refrigerant use, such as efficiency modifications or the banning of all refrigerants, which do not accurately capture the primary meaning of "substitute" in this context.