Which of the following is the smallest microorganism?

Options

A. Alga
B. Bacterium
C. Protozoan
D. Virus

Explanation

**Step-by-Step Solution:**

1. **Understanding the Question**: The question asks to identify the smallest microorganism from the given options: Algae, Bacterium, Protozoan, and Virus.

2. **Identifying the Organisms**:
- **Algae**: These are simple, typically autotrophic organisms that can be unicellular or multicellular.
- **Bacterium**: These are unicellular prokaryotic organisms, generally larger than viruses.
- **Protozoan**: These are unicellular eukaryotic organisms, larger than bacteria.
- **Virus**: These are acellular infectious agents that can only replicate inside a host cell.

3. **Comparing Sizes**:
- Algae, bacteria, and protozoans are all cellular organisms, while viruses are non-cellular.
- Among these, viruses are known to be significantly smaller than bacteria, protozoans, and algae.

4. **Conclusion**: Based on the comparison, the smallest microorganism among the options provided is the **Virus**.

5. **Final Answer**: The smallest microorganism is **Virus**.

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