Which one of the following is a bio indicator of air pollution?

Lichen is the right answer. It is a symbiotic association of a fungus and an alga or cyanobacterium. Lichens lack true roots and a waxy cuticle, so they take in water, nutrients and pollutants directly from air and rain. Sulphur dioxide is especially damaging to them, so their presence or absence in an area is a reliable indicator of air quality. Lichen-free zones around industrial towns are a classic sign of SO2 pollution.

Why the others fail:

• Fern: some ferns show sensitivity to pollutants, but they are not the standard bioindicator taught for air pollution monitoring.
• Money plant: it is popularly promoted as an indoor air purifier, but that is a claim about absorption, not a bioindicator of pollution levels.
• Dodder (Cuscuta): a parasitic plant with no established role in pollution monitoring.

Core concept: a bioindicator shows a visible or measurable response to environmental change, while a bioaccumulator stores pollutants. Lichens do both to some extent. For Mains, this links to urban air quality monitoring and to how ecological indicators support the standards framework under the Air (Prevention and Control of Pollution) Act, 1981.

Trap: confusing a bioindicator with a plant said to purify air. Ask whether the organism shows a measurable response to pollution, not whether it removes pollution.