An antitranspirant is

Options

A. Pheny mercuric acid
B. Abscisic acid
C. Salicylic acid
D. All the above

Answer

### Step-by-Step Solution:

1. **Understanding Transpiration**:
- Transpiration is the process by which water vapor is lost from the aerial parts of plants, primarily through small openings called stomata on the leaves.

2. **Importance of Transpiration**:
- This process is crucial for regulating plant temperature, facilitating the uptake of water and minerals from the soil, and is essential for photosynthesis.

3. **Consequences of Increased Transpiration**:
- If transpiration rates are too high, it can hinder plant growth and waste energy that would otherwise be used for water absorption and conduction.

4. **Definition of Antitranspirants**:
- Antitranspirants are substances that reduce the rate of transpiration. They work by closing the stomata, thereby minimizing water loss.

5. **Examples of Antitranspirants**:
- **Phenylmercury Acid (PMA)**: This chemical can be sprayed on plants to reduce transpiration.
- **Abscisic Acid**: A naturally occurring plant hormone that promotes the closure of stomata during water stress, thus reducing water loss.
- **Salicylic Acid**: Another plant hormone that can also function as an antitranspirant.

6. **Conclusion**:
- Since all the mentioned substances (phenylmercury acid, abscisic acid, and salicylic acid) can act as antitranspirants, the correct answer to the question is "all of the above".

### Final Answer:
An antitranspirant is **all of the above** (phenylmercury acid, abscisic acid, and salicylic acid).

---