The bioindicators of air pollution are

Options

A. algae
B. ferns
C. mushrooms
D. lichens

Explanation

### Step-by-Step Solution:

1. **Understanding Bioindicators**: Bioindicators are organisms that reflect the health of an environment. They are sensitive to changes in environmental conditions, particularly pollution levels. Their presence or absence can indicate the level of pollution in an area.

2. **Characteristics of Bioindicators**: Bioindicators are typically species that cannot survive in polluted environments. When pollution is present, their populations decline or disappear. Conversely, in clean environments, these organisms thrive and are found in large numbers.

3. **Example of a Bioindicator**: One of the most well-known bioindicators of air pollution is lichen. Lichens are symbiotic organisms made up of fungi and algae. The fungi provide a protective structure for the algae, while the algae produce food through photosynthesis.

4. **Sensitivity to Pollution**: Lichens are particularly sensitive to sulfur dioxide (SO2) gas, a common pollutant. When SO2 levels are high, lichens cannot survive, leading to a noticeable decline in their populations in polluted areas.

5. **Conclusion**: The presence of lichens in an area indicates that the environment is relatively clean, while their absence suggests pollution. Therefore, lichens serve as effective bioindicators of air pollution.