The sexual reproduction is absent in

Options

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

Answer

To solve the question "The sexual reproduction is absent in," we will analyze each option provided in the context of biological classification, specifically focusing on the type of organisms and their reproductive methods.

### Step-by-Step Solution:

1. **Identify the Organisms in the Options:**
- The options provided are Pyrogyra, Nostoc, Ulothrix, and Volvox.

2. **Analyze Pyrogyra:**
- Pyrogyra is a type of green algae.
- Green algae are eukaryotic organisms, and they typically reproduce sexually.
- **Conclusion:** Sexual reproduction is present in Pyrogyra.

3. **Analyze Nostoc:**
- Nostoc is classified as blue-green algae, which are also known as cyanobacteria.
- Cyanobacteria are prokaryotic organisms, meaning they lack a defined nucleus.
- Prokaryotic organisms do not undergo sexual reproduction in the traditional sense.
- **Conclusion:** Sexual reproduction is absent in Nostoc.

4. **Analyze Ulothrix:**
- Ulothrix is another type of green algae.
- Like Pyrogyra, Ulothrix is a eukaryotic organism and can reproduce sexually.
- **Conclusion:** Sexual reproduction is present in Ulothrix.

5. **Analyze Volvox:**
- Volvox is also a type of green algae and is eukaryotic.
- Volvox exhibits sexual reproduction as part of its life cycle.
- **Conclusion:** Sexual reproduction is present in Volvox.

6. **Final Conclusion:**
- Based on the analysis, the only organism among the options where sexual reproduction is absent is Nostoc (blue-green algae).

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