A unicellular microbe is:

Options

A. Spirogyra
B. Virus
C. Paramecium
D. Hydra

Explanation

To determine which of the given options is a unicellular microbe, we will analyze each option step by step.

### Step-by-Step Solution:

1. **Identify the Options**: The options provided are:
- Spirogyra
- Virus
- Paramecium
- Hydra

2. **Analyze Spirogyra**:
- Spirogyra is a type of green algae.
- It is known to be multicellular, as it forms filaments made up of many cells.
- **Conclusion**: Spirogyra is NOT a unicellular microbe.

3. **Analyze Virus**:
- Viruses are microscopic entities that can infect living organisms.
- They do not have a cellular structure; they consist of genetic material (DNA or RNA) surrounded by a protein coat.
- **Conclusion**: While viruses are microscopic, they are NOT considered unicellular microbes because they lack cellular machinery.

4. **Analyze Paramecium**:
- Paramecium is a type of protozoan, which belongs to the kingdom Protista.
- It is a single-celled organism and is classified as a ciliated protozoan.
- **Conclusion**: Paramecium is a unicellular microbe.

5. **Analyze Hydra**:
- Hydra is a multicellular organism that belongs to the phylum Cnidaria.
- It is a member of the animal kingdom and consists of multiple cells.
- **Conclusion**: Hydra is NOT a unicellular microbe.

6. **Final Conclusion**:
- The only option that qualifies as a unicellular microbe is **Paramecium**.

### Answer:
The unicellular microbe among the options is **Paramecium**.