Continued self-pollination result in inbreeding depression. Flowering plants have developed many devices to discourage self-pollination and to encourage pollination.

Cross-pollination is the transfer of pollen from the anther of one flower to the stigma of another flower. It plays a crucial role in the reproduction of many plant species as it brings about genetic variation by combining the genetic material from different individuals. In nature, there are various mechanisms that encourage cross-pollination. Among the options given, option A is the correct answer as it describes the device by which cross-pollination is encouraged in nature.

Self-incompatibility and mismatched timings for release of pollens and receptivity by stigma:
- Self-incompatibility is a mechanism by which a plant can prevent self-fertilization or self-pollination.
- Some plants have biochemical mechanisms that prevent the pollen from the same plant from germinating on the stigma of the same plant.
- This ensures that the plant is more likely to receive pollen from a different individual, promoting cross-pollination.
- Additionally, the timing of pollen release and stigma receptivity can be different in some plants, further reducing the chances of self-pollination and encouraging cross-pollination.

Pollen release and stigma receptivity are not synchronized:
- In some plant species, the release of pollen and the receptivity of the stigma do not occur at the same time.
- This is known as dichogamy and can be of two types: protandry and protogyny.
- Protandry refers to the condition where the anthers release pollen before the stigma is receptive.
- Protogyny, on the other hand, refers to the condition where the stigma becomes receptive before the anthers release pollen.
- These temporal differences promote cross-pollination as it increases the likelihood of pollen from one flower reaching the receptive stigma of another flower.

Self-incompatibility:
- Self-incompatibility is a genetic mechanism in plants that prevents self-fertilization.
- It can be controlled by a complex set of genes that recognize and reject pollen from the same plant.
- This mechanism ensures that plants receive pollen from a different individual, promoting cross-pollination.

In conclusion, the device by which cross-pollination is encouraged in nature is a combination of self-incompatibility and the mismatched timings for the release of pollens and receptivity by the stigma. These mechanisms ensure that plants receive pollen from different individuals, promoting genetic diversity and cross-pollination.