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

Options

A. Monoecious both autogamy and geitonogamy
B. Dioecious plant bearing only male or female flowers
C. Monoecious plant with bisexual flowers
D. Dioecious plant with bisexual flowers

Explanation

To solve the question of which situation prevents both autogamy and geitonogamy, we need to understand the definitions of these terms and analyze the options provided.

### Step-by-Step Solution:

1. **Define Autogamy and Geitonogamy**:
- **Autogamy**: This is a form of self-pollination where pollen from the anther of a flower fertilizes the stigma of the same flower. This occurs within the same flower of a plant.
- **Geitonogamy**: This is also a form of self-pollination but occurs between different flowers of the same plant. Pollen from the anther of one flower fertilizes the stigma of another flower on the same plant.

2. **Analyze the Options**:
- **Option A: Monoecious plants**: These plants have both male and female flowers on the same plant. Since both types of flowers are present, autogamy and geitonogamy can occur. Thus, this option does not prevent self-pollination.
- **Option B: Dioecious plants bearing only male or female flowers**: These plants have either male or female flowers on separate plants. Since there are no male and female flowers on the same plant, both autogamy and geitonogamy are prevented. This option is likely correct.
- **Option C: Monoecious plants with bisexual flowers**: Similar to option A, these plants have both male and female reproductive parts in the same flower, allowing for autogamy and potentially geitonogamy. Therefore, this option does not prevent self-pollination.
- **Option D: Dioecious plants with bisexual flowers**: This option is contradictory because dioecious plants cannot have bisexual flowers; they are characterized by having either male or female flowers. Therefore, this option is invalid.

3. **Conclusion**:
- The only situation that prevents both autogamy and geitonogamy is **Option B: Dioecious plants bearing only male or female flowers**. Since these plants do not have both types of flowers on the same plant, self-pollination cannot occur.

### Final Answer:
**Option B: Dioecious plants bearing only male or female flowers.**

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