The correct answer is option 2, Ozone layer. The Montreal Protocol is an international environmental agreement aimed at protecting the ozone layer. It was signed in 1987 and has been ratified by almost all countries. The ozone layer is a region in the Earth`s stratosphere that contains a high concentration of ozone gas, which plays a crucial role in absorbing the majority of the Sun`s ultraviolet (UV) radiation. However, the use of certain chemicals, such as chlorofluorocarbons (CFCs), was found to be depleting the ozone layer. The Montreal Protocol seeks to phase out the production and consumption of these harmful substances by implementing strict regulations and encouraging the use of ozone-friendly alternatives. Its successful implementation has significantly contributed to the recovery of the ozone layer.