[A]: Antitranspirants are material applied to plants for retarding transpiration.
[R]: Abscisic acid and phe_nyl mercuric ac- etate are not antitranspirants.

Options

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

Answer

### Step-by-Step Solution:

1. **Understanding the Assertion (A)**:
- The assertion states that antitranspirants are materials applied to plants to retard transpiration.
- Transpiration is the process by which plants lose water in the form of vapor, primarily through small openings called stomata.

2. **Understanding the Role of Antitranspirants**:
- Antitranspirants are substances that help reduce the rate of transpiration by closing the stomata, thus minimizing water loss from the plant.
- This is important for plants, especially in conditions where water conservation is crucial.

3. **Analyzing the Reason (R)**:
- The reason provided states that abscisic acid and phenyl mercuric acetate are not antitranspirants.
- Abscisic acid is a plant hormone known to play a role in closing stomata, thereby reducing transpiration. Therefore, it functions as an antitranspirant.
- Phenyl mercuric acetate is also known to have antitranspirant properties.

4. **Determining the Truth of A and R**:
- The assertion (A) is true because antitranspirants do indeed retard transpiration.
- The reason (R) is false because both abscisic acid and phenyl mercuric acetate are considered antitranspirants.

5. **Conclusion**:
- Since the assertion is true and the reason is false, the correct answer is that A is true and R is false.

### Final Answer:
The correct answer is: **Option 3: A is true, R is false.**

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