Final answer:
The Montreal Protocol aims to phase out the use of CFCs, contributing to the recovery of the ozone layer, although challenges such as illegal emissions and CFC 'banks' remain.
Step-by-step explanation:
The activity that the Montreal Protocol works to phase out is using CFCs (chlorofluorocarbons). This international treaty was established in 1987 with the aim to eliminate the production and consumption of substances that contribute to ozone layer depletion, primarily CFCs. The Protocol has been successful over the years, as seen in the recovery of the ozone layer and the reduction of ozone holes. However, there is still need for vigilance as illegal CFC emissions and the presence of CFC "banks"—where old equipment releases CFCs—pose risks to this progress.