An unicellular alga is

Options

A. Nostoc
B. Spirogyra
C. Chlamydomonas
D. Vaucheria

Explanation

**Step-by-Step Solution:**

1. **Identify the Options**: The question presents four options for unicellular algae: Nostoc, Spirogyra, Chlamydomonas, and Voucheria.

2. **Analyze Each Option**:
- **Nostoc**: This is a multicellular form of algae that is known for fixing atmospheric nitrogen. Therefore, it cannot be classified as unicellular.
- **Spirogyra**: This is also a multicellular alga characterized by its long filamentous structure. Hence, it is not unicellular.
- **Chlamydomonas**: This is a well-known unicellular alga. It consists of a single cell that can live in aquatic environments. Chlamydomonas has flagella for movement and can perform photosynthesis due to the presence of chlorophyll.
- **Voucheria**: Similar to Nostoc and Spirogyra, Voucheria is a multicellular alga.

3. **Conclusion**: After analyzing all the options, the only unicellular alga among the given choices is **Chlamydomonas**.

4. **Final Answer**: The unicellular alga is **Chlamydomonas**.

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