Assertion (A) : Cyanobacteria increases the soil fertility and also add the organic matter to the soil. Reason (R) : They can fix free nitrogen in the atmosphere.

Options

A. Both assertion (A) and reason (R) are true and reason (R) is the correct explanation of assertion (A).
B. Both assertion (A) and reason (R) are true but reason (R) is not the correct explanation of assertion (A).
C. Assertion (A) is true but reason (R) is false.
D. Assertion (A) is false but reason (R) is true.

Explanation

### Step-by-Step Solution:

1. **Understanding the Assertion (A)**:
- The assertion states that "Cyanobacteria increases the soil fertility and also adds organic matter to the soil."
- Cyanobacteria are known to be autotrophic microorganisms that can produce their own food through photosynthesis. They play a significant role in enhancing soil fertility.

2. **Understanding the Reason (R)**:
- The reason given is "They can fix free nitrogen in the atmosphere."
- Cyanobacteria have the ability to convert atmospheric nitrogen (N2) into a form that plants can use, such as ammonia (NH3). This process is known as nitrogen fixation.

3. **Linking Assertion and Reason**:
- The ability of cyanobacteria to fix nitrogen contributes to soil fertility because it increases the availability of nitrogen, an essential nutrient for plant growth.
- Additionally, as cyanobacteria grow and die, they contribute organic matter to the soil, further enhancing its fertility.

4. **Evaluating the Truth of Assertion and Reason**:
- Both the assertion and the reason are true. Cyanobacteria do indeed increase soil fertility and add organic matter due to their nitrogen-fixing capabilities.

5. **Determining the Correct Option**:
- Since both the assertion and reason are true, and the reason correctly explains why the assertion is true, the correct option is:
- **Option 1**: Both the assertion and the reason are true, and the reason is the correct explanation of the assertion.

### Final Answer:
Both the assertion and the reason are true, and the reason is the correct explanation of the assertion.

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