The correct answer is option 3: Depletion of Ozone layer. The Montreal Protocol is an international treaty that was created to address the issue of ozone depletion in the Earth`s atmosphere. The depletion of the ozone layer is primarily caused by the release of certain substances, such as chlorofluorocarbons (CFCs), which were commonly used in aerosol products and refrigerants.

The Montreal Protocol aims to reduce and eventually eliminate the use of these ozone-depleting substances. It sets out specific guidelines and targets for the phasing out of the production and consumption of these harmful substances. The protocol has been successful in reducing the use of CFCs and other ozone-depleting chemicals, leading to the recovery of the ozone layer.

It is important to note that the Montreal Protocol specifically addresses the issue of ozone depletion and does not deal with other subjects such as undersea cable networks, international cooperation against terrorism, or the exploration of outer space. Therefore, the correct answer is option 3, and the Montreal Protocol mainly deals with the depletion of the ozone layer.