Well defined nucleus is absent in

Options

A. blue-green algae
B. diatoms
C. algae
D. yeast

Explanation

### Step-by-Step Solution:

1. **Understanding the Question**: The question asks about the absence of a well-defined nucleus in certain organisms.

2. **Defining Nucleus Types**:
- A well-defined nucleus is characteristic of **Eukaryotic** cells, which have a nuclear membrane and well-organized nuclear structures.
- In contrast, **Prokaryotic** cells lack a well-defined nucleus; instead, they have a region called the **nucleoid** where the genetic material is located.

3. **Identifying Prokaryotes**:
- Prokaryotes include organisms like bacteria and cyanobacteria (blue-green algae). They do not have a nuclear membrane or well-defined nucleus.

4. **Analyzing the Options**:
- The options given are:
- Blue-green algae
- Diatoms
- Algae
- Yeast
- Among these, blue-green algae (cyanobacteria) are classified as prokaryotes.

5. **Classifying Other Options**:
- Diatoms, algae, and yeast are all eukaryotic organisms. They possess a well-defined nucleus and membrane-bound organelles.

6. **Conclusion**:
- Since blue-green algae are prokaryotic and lack a well-defined nucleus, the correct answer to the question is **blue-green algae**.

### Final Answer:
The well-defined nucleus is absent in **blue-green algae**.