The devices to discourage self pollination are

Options

A. pollen release and stigma receptivity is not syncronised
B. anther and stigma are placed at different position
C. rejection of pollen by stigma of the same flowers
D. all the these

Answer

**Step-by-Step Solution:**

1. **Understanding Self-Pollination and Cross-Pollination:**
- Self-pollination occurs when pollen from the same flower fertilizes its ovules, while cross-pollination involves pollen from one flower fertilizing the ovules of another flower.

2. **Identifying Devices to Discourage Self-Pollination:**
- To promote cross-pollination, plants have developed certain mechanisms or devices that discourage self-pollination.

3. **Analyzing the Given Options:**
- The options provided are:
a. Pollen release and stigma receptivity not synchronized.
b. Anthers and stigmas placed at different positions.
c. Rejection of pollen by the stigma of the same flower.
d. All of these.

4. **Evaluating Each Option:**
- **Option A:** If pollen release and stigma receptivity are not synchronized, it means that the pollen and stigma are not mature at the same time, making self-pollination less likely. This is a valid device to discourage self-pollination.
- **Option B:** If anthers and stigmas are placed at different positions, it makes it more difficult for pollen to reach the stigma of the same flower, thus promoting cross-pollination. This is also a valid device.
- **Option C:** The rejection of pollen by the stigma of the same flower means that even if pollen lands on the stigma, it may not germinate, thus preventing self-pollination. This is another valid device.

5. **Conclusion:**
- Since all three options (A, B, and C) are valid mechanisms that discourage self-pollination and encourage cross-pollination, the correct answer is **Option D: All of these**.