Assertion (A): Guard cells regulate the rate of transpiration in plants.
Reason (R): They are found enclosing the opening through which transpiration takes place.

Options

A. Both Assertion and Reason are true and Reason is the correct explanation for Assertion.
B. Both Assertion and Reason are true, and Reason is the not the correct explanation for Assertion.
C. Assertion is true and Reason is false.
D. Assertion is false and Reason is true.

Answer

### Step-by-Step Solution:

1. **Understanding the Assertion (A)**: The assertion states that "Guard cells regulate the rate of transpiration in plants." To analyze this, we need to know what guard cells are and their function in plants.

2. **Defining Transpiration**: Transpiration is the process by which water vapor is released from the plant into the atmosphere, primarily through small openings called stomata.

3. **Role of Guard Cells**: Guard cells are specialized cells that surround each stoma (the plural of stomata). Their main function is to control the opening and closing of the stomata.

4. **Regulating Transpiration**: When guard cells swell with water, they open the stomata, allowing water vapor to escape, which increases the rate of transpiration. Conversely, when they lose water, they become flaccid and close the stomata, reducing transpiration.

5. **Understanding the Reason (R)**: The reason states that "They are found enclosing the opening through which transpiration takes place." This refers to the stomata, which are indeed surrounded by guard cells.

6. **Evaluating the Relationship**: Since guard cells control the opening and closing of stomata, and stomata are the openings through which transpiration occurs, the reason provides a correct explanation for the assertion.

7. **Conclusion**: Both the assertion and the reason are true, and the reason correctly explains the assertion. Therefore, the answer is that both A and R are true, and R is the correct explanation of A.

### Final Answer:
Both Assertion (A) and Reason (R) are true, and R is the correct explanation of A.

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