The plant hormone is:

Options

A. insulin
B. thyroxine
C. oestrogen
D. cytokinin

Explanation

To determine which of the given options is a plant hormone, we can analyze each option step by step:

1. **Identify the Options**: The options provided are Insulin, Thyroxine, Estrogen, and Cytokinin.

2. **Analyze Insulin**:
- Insulin is a hormone produced in animals, specifically in the pancreas.
- Its primary function is to regulate blood glucose levels in the body.
- Conclusion: Insulin is not a plant hormone.

3. **Analyze Thyroxine**:
- Thyroxine is a hormone produced by the thyroid gland in animals.
- It plays a crucial role in regulating metabolism and growth.
- Conclusion: Thyroxine is not a plant hormone.

4. **Analyze Estrogen**:
- Estrogen is a hormone primarily associated with female reproductive functions in animals.
- It is involved in the regulation of the menstrual cycle and reproductive system.
- Conclusion: Estrogen is not a plant hormone.

5. **Analyze Cytokinin**:
- Cytokinin is a plant hormone that promotes cell division and growth in plants.
- It plays a significant role in various plant processes, including shoot and root development.
- Conclusion: Cytokinin is indeed a plant hormone.

6. **Final Answer**: Based on the analysis, the correct answer is Cytokinin, which is the only option that is a plant hormone.