Depletion of the ozone layer is mainly due to (a) chlorofluorocarbon compounds.
Explanation: Chlorofluorocarbons (CFCs), often used in refrigerants and aerosols, release chlorine atoms when they reach the stratosphere, which then react with ozone molecules and break them down, causing ozone depletion.
Why other options are incorrect:
(b) Carbon monoxide:
While carbon monoxide is a pollutant, it is primarily associated with air pollution and greenhouse gas emissions, not ozone depletion.
(c) Methane:
Methane is a greenhouse gas, contributing to global warming but not directly causing ozone depletion.
(d) Pesticides:
Pesticides can have various environmental impacts, but they are not the primary cause of ozone depletion.