Which of the following is a symbiotic nitrogen fixer ?

Options

A. Frankia
B. Azolla
C. Glomus
D. Azotobacter

Explanation

To determine which of the following is a symbiotic nitrogen fixer, we will analyze each option provided:

1. **Understanding Symbiotic Nitrogen Fixers**: Symbiotic nitrogen fixers are organisms that live in a close association with plants and help in converting atmospheric nitrogen into a form that plants can use. Common examples include certain bacteria that form symbiotic relationships with the roots of legumes.

2. **Analyzing the Options**:
- **Option A: Francia** - Francia is a genus of bacteria that forms a symbiotic relationship with certain plants, particularly in the root nodules. It is known for its role in nitrogen fixation.
- **Option B: Azolla** - Azolla is an aquatic fern that has a symbiotic relationship with cyanobacteria (like Anabaena) which can fix nitrogen. However, Azolla itself is not a nitrogen fixer but rather a host for nitrogen-fixing cyanobacteria.
- **Option C: Glomus** - Glomus is a genus of fungi that forms arbuscular mycorrhizal associations with plant roots. While they help plants absorb nutrients, they do not fix nitrogen themselves.
- **Option D: Azotobacter** - Azotobacter is a free-living nitrogen-fixing bacterium. It does not form a symbiotic relationship with plants but can fix nitrogen in the soil.

3. **Conclusion**: Based on the analysis, the correct answer is **Option A: Francia**, as it is a symbiotic nitrogen fixer.

### Step-by-Step Solution:
1. Identify the term "symbiotic nitrogen fixer."
2. Analyze each option provided:
- Francia: Symbiotic nitrogen fixer.
- Azolla: Aquatic fern, not a nitrogen fixer itself.
- Glomus: Arbuscular mycorrhizal fungi, not a nitrogen fixer.
- Azotobacter: Free-living nitrogen fixer, not symbiotic.
3. Conclude that the correct answer is **Francia**.