Triple response is shown by hormone

Options

A. Ethylene
B. CK
C. 2, 4-D
D. `GA_(3)`

Answer

### Step-by-Step Solution:

1. **Understanding the Question**: The question asks which hormone is responsible for the "triple response" in plants.

2. **Defining the Triple Response**: The triple response is a phenomenon observed in plants, particularly in seedlings, where they exhibit three specific growth responses when they encounter obstacles. These responses include:
- **Inhibition of stem elongation**: The plant slows down its vertical growth.
- **Thickening of the stem**: The stem becomes thicker to provide more support.
- **Curvature of the stem**: The stem bends to grow around the obstacle.

3. **Identifying the Hormone**: The hormone responsible for the triple response is **ethylene**. Ethylene is a gaseous plant hormone that plays a crucial role in regulating various aspects of plant growth and development.

4. **Mechanism of Ethylene**: Ethylene works by:
- Inhibiting the elongation of the hypocotyl (the part of the seedling below the cotyledons).
- Promoting the formation of the apical hook, which helps the seedling navigate around obstacles.

5. **Evaluating Other Hormones**:
- **Cytokinins (CK)**: These hormones are primarily involved in promoting cell division and are not responsible for the triple response.
- **2,4-Dichlorophenoxyacetic acid (2,4-D)**: This is a synthetic herbicide and does not play a role in the triple response.
- **Gibberellic acid (GA3)**: This hormone is involved in promoting seed germination and stem elongation, but not the triple response.

6. **Conclusion**: Based on the above analysis, the hormone that induces the triple response is **ethylene**.

### Final Answer:
The hormone responsible for the triple response is **ethylene**.