Leaf abscission is prevented by

Options

A. Gibberellin
B. Ethylene
C. Abscisic acid
D. Auxin.

Answer

### Step-by-Step Solution:

1. **Understanding Leaf Abscission**:
- Leaf abscission refers to the process where leaves fall off from a plant. This can occur due to various factors, including environmental stress or the natural life cycle of the plant.

2. **Identifying Plant Growth Regulators**:
- There are several plant growth regulators (hormones) that influence plant growth and development, including auxins, gibberellins, ethylene, and abscisic acid.

3. **Role of Auxin**:
- Auxins are a class of plant hormones that play a crucial role in various growth processes. One of their significant functions is to prevent leaf abscission, thereby keeping the leaves attached to the plant for a longer duration.

4. **Evaluating Other Options**:
- **Gibberellin**: This hormone is primarily involved in promoting cell elongation and does not prevent leaf abscission.
- **Ethylene**: This hormone actually promotes leaf abscission, making it an antagonist to auxin.
- **Abscisic Acid**: This hormone is known for its inhibitory effects, such as inhibiting seed germination, but it does not prevent leaf abscission.

5. **Conclusion**:
- Based on the roles of these hormones, the correct answer to the question "Leaf abscission is prevented by" is **Auxin**.

### Final Answer:
Leaf abscission is prevented by **Auxin**.

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