IAA is not the only naturally occurring auxin.

Explanation

**Step-by-Step Solution:**

1. **Understanding Auxins**: Auxins are a class of plant hormones that play a crucial role in the growth and development of plants. They are primarily responsible for promoting cell elongation, particularly in the stems.

2. **Identifying IAA**: Indole-3 Acetic Acid (IAA) is the most well-known and studied auxin. It is produced in the growing tips of shoots and roots and is vital for various growth processes.

3. **Recognizing Other Auxins**: While IAA is the primary auxin, it is important to note that it is not the only naturally occurring auxin. Other auxins include:
- **Phenyl Acetic Acid (PAA)**: Another naturally occurring auxin that contributes to plant growth.
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