The sexual reproduction is absent in

Options

A. Spirogyra
B. Nostoc
C. Ulothrix
D. Volvox

Answer

To determine which organism among the given options does not exhibit sexual reproduction, we can analyze each option step by step.

### Step-by-Step Solution:

1. **Identify the Organisms**: The options provided are Spirogyra, Nostoc, Ulothrix, and Volvox.

2. **Classify the Organisms**:
- **Spirogyra**: This is a green alga that reproduces sexually through conjugation.
- **Nostoc**: This is a blue-green alga (cyanobacteria) that primarily reproduces asexually.
- **Ulothrix**: This is also a green alga that can reproduce sexually.
- **Volvox**: This is a colonial green alga that reproduces both asexually and sexually.

3. **Analyze Sexual Reproduction**:
- **Spirogyra**: Engages in sexual reproduction.
- **Nostoc**: Does not engage in sexual reproduction; it reproduces asexually through vegetative methods.
- **Ulothrix**: Engages in sexual reproduction.
- **Volvox**: Engages in sexual reproduction.

4. **Conclusion**: Since Nostoc is the only organism listed that does not exhibit sexual reproduction, the correct answer is **Nostoc**.

### Final Answer:
The sexual reproduction is absent in **Nostoc**.

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