Assertion: Deficiency symptoms appear when the availability of the essential nutrients falls below the critical concentration.
Reason: Critical concentration is that limited concentration of the essential element below which growth of the plant is reduced.

Options

A. If both assertion and reason are true and reason is the correct explanation of assertion
B. if both assertion and reason are true but reason is not the correct explanation of assertion
C. if assertion is true but reason is false
D. if both assertion and reason are false

Answer

### Step-by-Step Solution:

1. **Understanding the Assertion**: The assertion states that deficiency symptoms appear when the availability of essential nutrients falls below the critical concentration. This means that if a plant does not receive enough of a necessary nutrient, it will start showing signs of deficiency.

2. **Understanding the Reason**: The reason provided explains what critical concentration is. It defines critical concentration as the minimum amount of an essential nutrient that a plant needs for normal growth. If the nutrient concentration falls below this level, the growth of the plant is negatively affected.

3. **Connecting Assertion and Reason**: The assertion and reason are closely related. The assertion indicates that deficiency symptoms will manifest when nutrient levels drop below a certain threshold (critical concentration). The reason explains that this threshold is the critical concentration itself, which is vital for plant growth.

4. **Evaluating the Truth of Both Statements**:
- The assertion is true because it accurately describes the relationship between nutrient availability and deficiency symptoms.
- The reason is also true as it correctly defines critical concentration and its impact on plant growth.

5. **Determining the Relationship**: Since both the assertion and reason are true, and the reason provides a correct explanation for the assertion, we conclude that the relationship between the two is valid.

6. **Final Conclusion**: Therefore, the correct answer is that both assertion and reason are true, and the reason is the correct explanation of the assertion.