The sexual reproduction is absent in

Options

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

Answer

To solve the question regarding which organism does not exhibit sexual reproduction among Eulothrix, Volvox, Spirogyra, and Nostoc, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Organisms**:
- **Spirogyra**: This is a filamentous green algae that can reproduce both sexually and asexually. Asexual reproduction occurs through fragmentation, while sexual reproduction occurs via conjugation.
- **Volvox**: This is a colonial green algae that can also reproduce both sexually and asexually. Asexual reproduction occurs through the formation of daughter colonies, while sexual reproduction involves the formation of gametes.
- **Eulothrix**: This is an unbranched filamentous green algae that can reproduce both asexually (through vegetative means) and sexually.
- **Nostoc**: This is a cyanobacterium (blue-green algae) that primarily reproduces asexually. It forms colonies and can produce structures like hormogonia and akinetes, but it does not have a recognized sexual reproduction process.

2. **Identify the Absence of Sexual Reproduction**:
- From the descriptions, we can conclude that Spirogyra, Volvox, and Eulothrix all have mechanisms for sexual reproduction. In contrast, Nostoc does not exhibit sexual reproduction.

3. **Conclusion**:
- Therefore, the correct answer to the question "The sexual reproduction is absent in" is **Nostoc**.

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