Air quality indicator is

Options

A. Lichen
B. Moss
C. Algae
D. None of these

Answer

### Step-by-Step Solution:

1. **Understanding the Question**: The question asks for an air quality indicator among the given options: lichen, mouse, algae, or none of these.

2. **Identifying the Correct Answer**: The correct answer is **lichen**. Lichens are known to be sensitive to air quality and are commonly used as bioindicators.

3. **Defining Lichen**: Lichen is not a single organism; it is a symbiotic association between two organisms: a fungus and an alga (or cyanobacteria). This unique relationship allows lichens to thrive in various environments.

4. **Role of Lichens as Air Quality Indicators**:
- Lichens absorb nutrients from the air rather than the soil. This means they are directly affected by the quality of the air around them.
- They accumulate pollutants, such as sulfur dioxide, from the atmosphere. When sulfur dioxide levels are high, it can lead to visible changes in the color and health of lichens.

5. **Detection of Other Pollutants**: Besides sulfur dioxide, lichens can also indicate the presence of radioactive elements in the air, making them effective bioindicators for various types of air pollution.

6. **Conclusion**: Therefore, the air quality indicator is **lichen**.