Apical dominance is not affected by
(a) Indoleacetic acid
(b) Gibberellins
(c) Indoleacetaldehyde
(d) Indolebutyric acid

Options

A. Indoleacetic acid
B. Gibberellins
C. Indoleacetaldehyde
D. Indolebutyric acid

Answer

To solve the question regarding apical dominance and which substance does not affect it, we can follow these steps:

### Step-by-Step Solution:

1. **Understand Apical Dominance**:
- Apical dominance is a phenomenon where the growth of the main (apical) shoot is prioritized over the growth of lateral (axillary) buds. This is primarily regulated by plant hormones known as auxins.

2. **Identify Auxins**:
- The options provided include:
- (a) Indoleacetic acid (IAA) - This is a well-known auxin.
- (b) Gibberellins - These are not auxins but are another class of plant hormones that promote growth, particularly stem elongation.
- (c) Indoleacetaldehyde - This is a precursor to auxins and can be considered related to auxin activity.
- (d) Indolebutyric acid (IBA) - This is also an auxin.

3. **Analyze the Options**:
- Options (a), (c), and (d) are all related to auxins and are known to influence apical dominance by promoting the growth of the apical bud and inhibiting lateral bud growth.
- Option (b), gibberellins, while it promotes growth, does not directly affect apical dominance in the same way that auxins do.

4. **Conclusion**:
- Since apical dominance is primarily influenced by auxins, and gibberellins do not have a direct effect on this phenomenon, the correct answer is (b) Gibberellins.

### Final Answer:
**(b) Gibberellins**