Which one of the plant growth regulators would you use if you are asked to
induce rooting in a twig.

Answer

To induce rooting in a twig, the appropriate plant growth regulator to use is auxin. Here’s a step-by-step explanation:

### Step-by-Step Solution:

1. **Understand the Purpose**: The question asks for a plant growth regulator that can induce rooting in a twig. This means we need to identify which hormone is responsible for promoting root development.

2. **Identify Plant Growth Regulators**: There are several plant growth regulators (hormones) that play different roles in plant growth and development. The main ones include:
- Auxins
- Gibberellins
- Cytokinins
- Ethylene
- Abscisic Acid

3. **Focus on Rooting**: Among these hormones, auxins are specifically known to promote root formation. They are often used in horticulture to encourage root development in cuttings.

4. **Eliminate Other Options**:
- Ethylene is known for promoting fruit ripening.
- Cytokinins are involved in cell division and delaying senescence (aging).
- Gibberellins promote stem elongation and seed germination.
- Abscisic acid is primarily involved in stress responses and stomatal closure.

5. **Conclusion**: Since auxin is the hormone that induces rooting, the answer to the question is auxin.

### Final Answer:
**Auxin** is the plant growth regulator used to induce rooting in a twig.