External fertilization is not observed in

Options

A. Yeast
B. Spirogyra
C. Chlamydomonas
D. Ulothrix

Answer

To solve the question "External fertilization is not observed in which of the following organisms?", we need to analyze the options provided and determine which organism exhibits internal fertilization instead.

### Step-by-Step Solution:

1. **Understand the Definitions**:
- **External Fertilization**: This occurs when the fusion of gametes (sperm and egg) happens outside the body of the organism. Common in aquatic animals like fish and amphibians.
- **Internal Fertilization**: This occurs when the fusion of gametes happens inside the body of the organism. Common in terrestrial animals, including mammals.

2. **Analyze the Options**:
- The options given are: Yeast, Spirogyra, Chlamydomonas, and Eulothrix.

3. **Evaluate Each Option**:
- **Yeast**: Yeast reproduces sexually through a process called budding or fission, and it does not involve external fertilization. Instead, it can undergo a form of internal fertilization.
- **Spirogyra**: This is a filamentous green algae that reproduces sexually through conjugation, which is a form of external fertilization.
- **Chlamydomonas**: This is a unicellular green algae that also reproduces sexually through external fertilization.
- **Eulothrix**: Similar to Spirogyra, Eulothrix reproduces through external fertilization.

4. **Identify the Correct Answer**:
- Among the options, only **Yeast** exhibits internal fertilization, while the others (Spirogyra, Chlamydomonas, and Eulothrix) exhibit external fertilization.

5. **Conclusion**:
- Therefore, the correct answer is **Yeast**, as external fertilization is not observed in this organism.

### Final Answer:
External fertilization is not observed in **Yeast**.