Oxygen evolving and nitrogen fixing organisms is

Options

A. Azotobacter
B. Nostoc
C. Frankia
D. Clostridium

Answer

**Step-by-Step Solution:**

1. **Understanding the Question**: The question asks for an organism that is both oxygen-evolving and nitrogen-fixing. This means we need to identify an organism that can produce oxygen (usually through photosynthesis) and also convert atmospheric nitrogen into a usable form for plants and animals.

2. **Identifying the Options**: The options given are:
- A) Azotobacter
- B) Nostoc
- C) Cyanobacteria
- D) Rhizobium

3. **Analyzing Each Option**:
- **A) Azotobacter**: This is a free-living nitrogen-fixing bacterium, but it does not produce oxygen.
- **B) Nostoc**: This is a genus of cyanobacteria that can fix nitrogen and also perform photosynthesis, thus evolving oxygen.
- **C) Cyanobacteria**: This is a broad category that includes organisms that can both fix nitrogen and produce oxygen.
- **D) Rhizobium**: This is a nitrogen-fixing bacterium that lives in symbiosis with legumes but does not produce oxygen.

4. **Selecting the Correct Answer**: Among the options, **Nostoc** is the most specific answer that fits the criteria of being both oxygen-evolving and nitrogen-fixing. Cyanobacteria as a group also fit, but Nostoc is a specific example.

5. **Conclusion**: Therefore, the correct answer to the question is **B) Nostoc**.

---