Situation that prevents both autogamy and geitonogamy: Dioecious plant bearing only male or female flowers (Option B)

1. Monoecious plant bearing unisexual flowers:
- In a monoecious plant, both male and female flowers are present on the same plant.
- Although the flowers are unisexual, both male and female flowers are present on the same plant, allowing for the possibility of autogamy and geitonogamy.
- Autogamy refers to the transfer of pollen from the anther to the stigma of the same flower, while geitonogamy refers to the transfer of pollen from the anther of one flower to the stigma of another flower on the same plant.
- Therefore, this situation does not prevent both autogamy and geitonogamy.

2. Dioecious plant with bisexual flowers:
- In a dioecious plant, male and female flowers are present on separate plants.
- If the flowers are bisexual, each flower has both male and female reproductive structures.
- This situation allows for the possibility of geitonogamy because the male and female flowers are present on different plants, but it does not prevent autogamy.
- Autogamy can still occur within each bisexual flower, where the pollen can be transferred from the anther to the stigma of the same flower.
- Therefore, this situation does not prevent both autogamy and geitonogamy.

3. Monoecious plant with bisexual flowers:
- In a monoecious plant, both male and female flowers are present on the same plant.
- If the flowers are bisexual, each flower has both male and female reproductive structures.
- This situation allows for the possibility of both autogamy and geitonogamy.
- Autogamy can occur within each bisexual flower, and geitonogamy can occur between the male and female flowers on the same plant.
- Therefore, this situation does not prevent both autogamy and geitonogamy.

4. Dioecious plant bearing only male or female flowers:
- In a dioecious plant, male and female flowers are present on separate plants.
- If the plant bears only male or female flowers, there is no possibility of autogamy or geitonogamy.
- Autogamy requires the presence of both male and female reproductive structures in the same flower, which is not possible in this situation.
- Geitonogamy requires the presence of both male and female flowers on the same plant, which is also not possible in this situation.
- Therefore, this situation prevents both autogamy and geitonogamy.

Therefore, the correct answer is option B: Dioecious plant bearing only male or female flowers.