Unicellular symbiotic organisms improve yield of legumes by

Options

A. fixing atmospheric nitrogen without colonising roots of host plant
B. fixing atmospheric nitrogen and colonising roots of host plant
C. inducing the host plant to absorb more phosphours
D. stimulating the host plant to become tolerant to drought

Explanation

To answer the question regarding how unicellular symbiotic organisms improve the yield of legumes, we can break down the information as follows:

### Step-by-Step Solution:

1. **Understanding Symbiotic Relationships**:
- Symbiotic organisms, particularly certain bacteria, form a mutualistic relationship with legume plants. In this relationship, both the bacteria and the plant benefit.

2. **Role of Nitrogen-Fixing Bacteria**:
- The most notable nitrogen-fixing bacteria associated with legumes are called Rhizobia. These bacteria are capable of converting atmospheric nitrogen into a form that plants can use.

3. **Colonization of Roots**:
- Rhizobia colonize the roots of legume plants and form specialized structures known as root nodules. Within these nodules, the bacteria can effectively fix nitrogen.

4. **Mutual Benefits**:
- In return for the nitrogen fixed by the bacteria, the legumes provide the bacteria with carbohydrates and a suitable environment to thrive, which includes shelter and nutrients.

5. **Analyzing the Options**:
- **Option A**: Fixing atmospheric nitrogen without colonizing roots of host plant - This is incorrect because Rhizobia must colonize the roots to fix nitrogen.
- **Option B**: Fixing atmospheric nitrogen and colonizing roots of host plant - This is correct as it describes the mutualistic relationship accurately.
- **Option C**: Inducing the host plant to absorb more phosphorus - This is not the primary function of these bacteria.
- **Option D**: Stimulating the host plant to become tolerant to drought - This is also not a direct function of the nitrogen-fixing bacteria.

6. **Conclusion**:
- The correct answer is **Option B**: fixing atmospheric nitrogen and colonizing roots of host plant.