In which of the following sexual reproduction is unknown ?

Options

A. Chlorella
B. Spirogyra
C. Chlamydomonas
D. Ulva.

Answer

To determine which of the following organisms does not exhibit sexual reproduction, we will analyze each option provided in the context of the phylum Chlorophyta, commonly known as green algae.

### Step-by-Step Solution:

1. **Identify the Organisms**: We need to look at the four options given in the question. These organisms belong to the phylum Chlorophyta.

2. **Understand Sexual Reproduction in Green Algae**:
- **Ulva**: This green algae reproduces sexually through the formation of zoospores. Therefore, sexual reproduction is present.
- **Chlamydomonas**: This organism reproduces sexually through isogamous gametes, meaning it can produce gametes that are morphologically similar. Hence, sexual reproduction is also present here.
- **Spirogyra**: This organism reproduces sexually through a process called conjugation, where two filaments align and exchange genetic material. Thus, sexual reproduction occurs in Spirogyra as well.
- **Chlorella**: This organism is known to reproduce asexually only, primarily through autospores, which are clones of the parent organism. Therefore, sexual reproduction is absent in Chlorella.

3. **Conclusion**: Based on the analysis, the organism in which sexual reproduction is unknown is **Chlorella**.

### Final Answer:
The correct answer is **Chlorella**.

---