How many of the following are natural plant growth regulators?
(1) kinetin (2) IAA (3) 2-4.D (4) IBA (5) NAA , (6) Zeatin (7) `GA_3`, (8) Ethephon

Options

A. Four
B. three
C. two
D. six

Answer

To determine how many of the listed substances are natural plant growth regulators, we will analyze each option one by one.

1. **Kinetin**: This is a synthetic cytokinin and not found naturally in plants.
- **Hint**: Check if the substance is synthesized or naturally occurring.

2. **IAA (Indole-3-acetic acid)**: This is a natural auxin and is produced in plants.
- **Hint**: Identify if the substance is a naturally occurring form of auxin.

3. **2,4-D (2,4-dichlorophenoxyacetic acid)**: This is a synthetic auxin used as a herbicide and does not occur naturally.
- **Hint**: Consider if the substance is used in agriculture and its origin.

4. **IBA (Indole-3-butyric acid)**: This is also a natural auxin found in plants.
- **Hint**: Look for compounds that are derivatives of natural plant hormones.

5. **NAA (1-naphthaleneacetic acid)**: This is a synthetic auxin and does not occur naturally.
- **Hint**: Determine if the substance is a synthetic analog of a natural hormone.

6. **Zeatin**: This is a naturally occurring cytokinin found in plants.
- **Hint**: Check if the substance is derived from natural plant processes.

7. **GA3 (Gibberellic acid)**: This is a natural plant growth regulator produced by fungi and plants.
- **Hint**: Identify if the substance is produced by living organisms.

8. **Ethephon**: This is a synthetic compound that releases ethylene and is not a natural growth regulator.
- **Hint**: Assess if the substance is a natural hormone or a synthetic compound that affects plant growth.

Now, let's summarize the findings:
- Natural plant growth regulators from the list:
- IAA (2)
- IBA (4)
- Zeatin (6)
- GA3 (7)

Thus, there are **4 natural plant growth regulators** in the provided list.

### Final Answer:
There are **4 natural plant growth regulators** from the given options.