The correct answer to the question is option 3: Active Cl. Ozone depletion in the stratosphere occurs due to the presence of certain active substances, primarily halogen compounds. These halogen compounds include chlorine (Cl), which is highly reactive and can destroy ozone molecules.

Chlorine compounds, such as chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs), were commonly used in refrigerants, aerosol propellants, and foam-blowing agents. When released into the atmosphere, these compounds can reach the stratosphere and undergo photolysis – a process where the compounds are broken down by sunlight. This leads to the release of chlorine atoms.

Once chlorine atoms are released into the stratosphere, they can engage in a catalytic cycle that results in the destruction of ozone. A chlorine atom can react with an ozone molecule, resulting in the formation of chlorine monoxide (ClO) and oxygen gas. ClO is also reactive and can react with another ozone molecule, producing more chlorine atoms and further depleting ozone.

It is worth mentioning that other halogens, such as bromine (Br), can also contribute to ozone depletion, but in smaller quantities compared to