Phototropism is due to the hormone
(a) IAA
(b) GA
(c) 2-4 D
(d) Cytokinin

Options

A. IAA
B. GA
C. 2-4 D
D. Cytokinin

Answer

### Step-by-Step Solution:

1. **Understanding Phototropism**:
- Phototropism refers to the growth movement of a plant in response to light direction. Plants tend to bend towards the light source, which is crucial for maximizing photosynthesis.

2. **Role of Hormones in Phototropism**:
- The bending of the plant towards light is regulated by plant hormones, specifically auxins. Auxins are responsible for promoting cell elongation on the side of the plant that is away from the light.

3. **Identifying the Correct Hormone**:
- Among the options provided:
- (a) IAA (Indole-3-acetic acid) is a type of auxin.
- (b) GA (Gibberellins) are involved in growth but not specifically in phototropism.
- (c) 2,4-D is a synthetic auxin used as a herbicide.
- (d) Cytokinins promote cell division but are not directly involved in phototropism.

4. **Conclusion**:
- Since phototropism is primarily influenced by auxins, and IAA is a natural auxin produced in plants, the correct answer is (a) IAA.

### Final Answer:
Phototropism is due to the hormone **(a) IAA**.

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