**Core Concept**
The question is testing the student's knowledge of environmental health and the effects of air pollution on living organisms. The best biological indicator of air pollution is an organism that can be used as a marker to measure the level of pollutants in the environment.

**Why the Correct Answer is Right**
The correct answer is Lichens. Lichens are composite organisms made up of fungi and algae or cyanobacteria. They are sensitive to air pollutants and can be used as a biological indicator of air pollution. Lichens are sensitive to sulfur dioxide, nitrogen oxides, and particulate matter, making them a good indicator of air quality. They can be found growing on rocks and trees in areas with low pollution levels, but as pollution increases, lichens tend to disappear. This makes them a useful tool for monitoring air pollution.

**Why Each Wrong Option is Incorrect**
**Option A:** is incorrect because it is not a specific type of organism that can be used as a biological indicator of air pollution.

**Option B:** is incorrect because, while trees can be affected by air pollution, they are not as sensitive as lichens and are not typically used as biological indicators of air pollution.

**Option C:** is incorrect because bacteria are not typically used as biological indicators of air pollution.

**Clinical Pearl / High-Yield Fact**
It's worth noting that lichens are not the only biological indicators of air pollution, but they are one of the most sensitive and commonly used. Other biological indicators of air pollution include mosses, ferns, and certain types of fungi.

**Correct Answer: C. Lichens**