Short day plant is

Options

A. Xanthium
B. Pisum
C. Cucumis
D. Avena.

Answer

### Step-by-Step Solution:

1. **Definition of Short Day Plants (SDP)**:
- Short day plants are defined as plants that initiate flowering when the day length is shorter than their critical day length. This means they require longer periods of darkness to trigger the flowering process.

2. **Characteristics of Short Day Plants**:
- These plants thrive in conditions where the duration of light is less than a certain threshold. They need a longer duration of darkness (night) compared to light (day) for flowering to occur.

3. **Examples of Short Day Plants**:
- Common examples of short day plants include:
- **Xanthium**
- **Rice**
- **Tobacco**
- **Cotton**

4. **Distinction from Long Day Plants**:
- In contrast, long day plants require longer daylight hours and shorter periods of darkness to flower. Examples of long day plants include Avena, Pisum, Cucumis, and certain weeds.

5. **Conclusion**:
- Based on the definition and examples provided, the correct answer to the question "Short day plant is" would be **Xanthium**.