Option 1: Ferns are not typically used as indicators of air pollution, as they do not show specific signs or symptoms in response to SO2 pollution. Alert - correct answer should be option 3: Lichens.

Option 2: Mentha, also known as mint, is not commonly used as an indicator of air pollution. It is a plant often grown for its culinary and aromatic purposes and does not exhibit specific responses to SO2 pollution.

Option 3: Lichens are commonly used as indicators of SO2 pollution in the air. Lichens are composed of a symbiotic relationship between fungi and algae and are highly sensitive to air pollution. They can be found growing on trees, rocks, and other surfaces, and their presence or absence can indicate the level of air quality in the area. Lichens are particularly sensitive to SO2 pollution, as high concentrations of this pollutant can cause their decline or disappearance.

Option 4: Homworts, also known as horsetails, are not commonly used as indicators of air pollution. They grow in a variety of environments and do not show specific responses to SO2 pollution.

In conclusion, lichens are a group of plants that can be used as indicators of SO2 pollution in the air