Explanation:

In plants, self-pollination or autogamy refers to the process where the transfer of pollen grains occurs from the anther to the stigma of the same flower or the same plant. This process ensures reproduction and seed production even in the absence of pollinators. There are several mechanisms by which self-pollination or autogamy can occur, and these mechanisms are collectively known as contrivances.

1. Geitonogamy:
Geitonogamy is a type of self-pollination that occurs within the same plant but between different flowers. In this process, pollen grains are transferred from the anther of one flower to the stigma of another flower on the same plant. This can happen through various means such as wind, water, or self-movement of floral parts. Geitonogamy helps in the transfer of genetic material within the same plant, ensuring reproductive success.

2. Homogamy:
Homogamy is another type of self-pollination that occurs within the same flower. In this process, the pollen grains are transferred from the anther to the stigma of the same flower. Homogamy can occur through different mechanisms such as self-pollination due to close proximity of the anther and stigma within the same flower, or through self-movement of floral parts. Homogamy is a reliable mechanism for self-pollination as it ensures the transfer of pollen within the same flower, reducing the chances of pollen loss.

3. Cleistogamy:
Cleistogamy is a unique mechanism of self-pollination where the flowers remain closed and do not open for pollination. In this process, the flowers self-pollinate without any external agents like pollinators. Cleistogamy is commonly observed in plants that grow in shady or dense habitats where pollinators are scarce. The flowers in cleistogamous plants produce small, inconspicuous flowers that self-pollinate within the closed environment. This ensures reproductive success even in the absence of pollinators.

Conclusion:
All of the above mechanisms, geitonogamy, homogamy, and cleistogamy, are contrivances for self-pollination or autogamy. These mechanisms ensure the transfer of pollen grains within the same flower or the same plant, allowing for reproduction and seed production even in the absence of external pollinators. By employing these contrivances, plants can ensure their survival and maintain genetic diversity within their populations. Therefore, the correct answer is option 'D' - all of the above.

Plants have specific adaptations or so-called contrivances for both autogamy and allogamy.