From among the situations given below, choose the one that prevents both autogamy and geitonogamy.

Options

A. Monoecious plant bearing unisexual flowers.
B. Dioecious plant bearing unisexual flowers.
C. Monocious plant with bisexual flowers.
D. Dioecious plant with bisexual flowers.

Explanation

To solve the question, "From among the situations given below, choose the one that prevents both autogamy and geitonogamy," we will analyze the definitions of autogamy and geitonogamy and evaluate the options provided.

### Step-by-Step Solution:

1. **Define Autogamy**:
- Autogamy is a type of self-pollination where pollen from the same flower falls on its stigma. This means that the plant can fertilize itself without the need for pollen from another flower.

2. **Define Geitonogamy**:
- Geitonogamy is also a form of self-pollination, but it involves pollen from one flower of a plant falling on the stigma of another flower of the same plant. This means that while the pollen comes from a different flower, it is still within the same plant.

3. **Understanding the Options**:
- **Option A**: Monoecious plant bearing unisexual flowers - This allows for geitonogamy since the unisexual flowers are on the same plant, but autogamy is not possible.
- **Option B**: Dioecious plant bearing unisexual flowers - This prevents both autogamy and geitonogamy because there are separate male and female plants; thus, self-pollination cannot occur.
- **Option C**: Monoecious plant with bisexual flowers - This allows for both autogamy and geitonogamy since the same plant has both male and female parts.
- **Option D**: Dioecious plant with bisexual flowers - This is contradictory because dioecious plants cannot have bisexual flowers.

4. **Conclusion**:
- The option that prevents both autogamy and geitonogamy is **Option B: Dioecious plant bearing unisexual flowers**. In this case, since the male and female flowers are on separate plants, neither form of self-pollination can occur.

### Final Answer:
**Option B: Dioecious plant bearing unisexual flowers.**