Some organisms are sensitive to different levels of air pollution and are used as pollution indicators. Suggest which among the following fits into the category.

Options

A. Fungi
B. Fresh water algae
C. Bacteria
D. Lichens

Explanation

To solve the question regarding which organisms are sensitive to different levels of air pollution and can be used as pollution indicators, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding Pollution Indicators**: Pollution indicators are organisms that are sensitive to changes in environmental conditions, particularly air quality. They can indicate the presence of pollutants in the environment.

2. **Analyzing the Options**: The options given are fungi, freshwater algae, bacteria, and lichen. We need to determine which of these organisms is most sensitive to air pollution.

3. **Researching Each Organism**:
- **Fungi**: Generally, fungi can tolerate a range of environmental conditions, including some levels of pollution.
- **Freshwater Algae**: While some algae can be sensitive to pollution, many species can thrive in polluted waters.
- **Bacteria**: Bacteria are highly adaptable and can survive in various conditions, including polluted environments.
- **Lichen**: Lichens are known to be very sensitive to air pollution, especially to nitrogen compounds and sulfur dioxide. They absorb nutrients and water from the atmosphere, making them vulnerable to changes in air quality.

4. **Conclusion**: Based on the sensitivity to air pollution, lichen is the most suitable organism that fits into the category of pollution indicators. They are often used to monitor air quality because their presence or absence can indicate the level of pollution in an area.

5. **Final Answer**: The correct answer is **lichen**.