Which of the following is known as stress hormone?

Options

A. Ethylene
B. Auxin
C. Abscisic Acid
D. `GA_(3)`

Answer

**Step-by-Step Solution:**

1. **Understanding the Question**: The question asks for the hormone that is known as the "stress hormone" among the provided options.

2. **Analyzing the Options**:
- **Option 1: Ethylene**: This is a gaseous hormone primarily associated with fruit ripening. It is not known as a stress hormone.
- **Option 2: Oxygen**: While oxygen is essential for plant respiration, it is not classified as a plant hormone, nor is it referred to as a stress hormone.
- **Option 3: Abscisic Acid**: This hormone is known as the "stress hormone" because it plays a crucial role in helping plants respond to stress conditions, such as drought. It stimulates the closure of stomata, which helps to conserve water.
- **Option 4: Gibberellic Acid**: This hormone is involved in promoting seed germination and growth, but it is not associated with stress responses.

3. **Conclusion**: After analyzing all the options, it is clear that Abscisic Acid is the hormone known as the stress hormone.

4. **Final Answer**: The correct answer is **Abscisic Acid**.

---