The most common auxin is
(a) GA
(b) ABA
(c) Kinetin
(d) IAA

Options

A. GA
B. ABA
C. Kinetin
D. IAA

Answer

To solve the question regarding the most common auxin, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Question**: The question asks for the most common auxin among the given options: GA, ABA, Kinetin, and IAA.

2. **Identify the Options**:
- (a) GA - Gibberellic Acid
- (b) ABA - Abscisic Acid
- (c) Kinetin - A type of cytokinin
- (d) IAA - Indole-3-acetic acid

3. **Define Auxins**: Auxins are a class of plant hormones that play a crucial role in the regulation of plant growth and development, particularly in cell elongation, root formation, and responses to light and gravity.

4. **Research Each Option**:
- **GA (Gibberellic Acid)**: This is primarily involved in promoting stem elongation, seed germination, and flowering but is not classified as an auxin.
- **ABA (Abscisic Acid)**: This hormone is involved in stress responses and regulation of stomatal closure, not auxin-related functions.
- **Kinetin**: This is a cytokinin, which promotes cell division and is not classified as an auxin.
- **IAA (Indole-3-acetic acid)**: This is the most common and principal auxin found in higher plants.

5. **Conclusion**: Based on the definitions and functions of the hormones listed, the most common auxin is **IAA (Indole-3-acetic acid)**.

6. **Final Answer**: Therefore, the correct answer is (d) IAA.