## Core Concept
The question pertains to environmental health and pollution indicators. **Biological indicators**, particularly those that reflect the health of ecosystems and can signal pollution levels, are crucial. Lichens are widely recognized for their sensitivity to environmental pollutants.

## Why the Correct Answer is Right
The correct answer, **C. Lichens**, is right because lichens are highly sensitive to air pollution, especially sulfur dioxide and nitrogen oxides. They have a unique symbiotic relationship between fungi and algae or cyanobacteria, which makes them excellent **bioindicators** of air quality. Their sensitivity to pollutants means they can thrive in clean environments but may die off or change composition in polluted areas, making them a practical and visible indicator of pollution levels.

## Why Each Wrong Option is Incorrect
- **Option A:** While certain types of algae can be indicators of water pollution, they are not as universally recognized or as broadly applicable as lichens for indicating general pollution levels, especially air pollution.
- **Option B:** Mosses can also serve as indicators of pollution, particularly for heavy metals and acid rain. However, they are not as sensitive or as widely used as indicators as lichens.
- **Option D:** Ferns can indicate certain types of environmental conditions and pollution, but they are not as directly associated with a broad range of pollutants or as commonly used for monitoring pollution as lichens.

## Clinical Pearl / High-Yield Fact
A memorable point for exams is that **lichens are often used in "lichenometry"**, a technique to assess the age of rocks and to study environmental changes over time. Their presence or absence can give clues about the level of pollution and environmental health.

## Correct Answer Line
**Correct Answer: C. Lichens.**