Which of the following is not an example of lichen?

Options

A. Cladonia
B. Usnea
C. Giardia
D. Both (a) and (b)

Explanation

**Step-by-Step Solution:**

1. **Understanding Lichens**:
- Lichens are a symbiotic association between fungi and algae. In this relationship, algae provide food through photosynthesis, while fungi offer protection and a suitable habitat for the algae.

2. **Identifying Examples of Lichens**:
- The question provides four options, and we need to determine which one is not a lichen.
- Common examples of lichens include Cladonia and Usnea. Both of these are known to thrive in clean, pollution-free environments and are often used as indicators of air quality.

3. **Analyzing the Options**:
- Cladonia: This is a type of lichen that is indeed a mutualistic relationship between fungi and algae.
- Usnea: This is another example of lichen, also a mutualistic relationship.
- Giardia: This is not a lichen; it is a genus of microorganisms known as parasites that cause diseases, particularly diarrhea in humans.

4. **Conclusion**:
- Since Giardia is a parasite and does not form a mutualistic relationship like lichens do, it is the correct answer to the question of which is not an example of lichen.

5. **Final Answer**:
- The option that is not an example of lichen is **Giardia**.

---