In Castor and Maize plants
(a) Autogamy is prevented but not geitonogamy
(b) Both autogamy and geitonogamy are prevented
(c) Male and female flowers are borne by different plants
(d) Anthers and stigma are placed at different positions to encourage cross pollination

Options

A. Autogamy is prevented but not geitonogamy
B. Both autogamy and geitonogamy are prevented
C. Male and female flowers are borne by different plants
D. Anthers and stigma are placed at different positions to encourage cross pollination.

Answer

To solve the question regarding the reproductive characteristics of Castor and Maize plants, we will analyze each option step by step.

### Step-by-Step Solution:

1. **Understanding the Flower Structure**:
- Castor and Maize plants have unisexual flowers, meaning that they produce either male or female flowers, but not both on the same flower. This is an important characteristic that influences their pollination methods.

**Hint**: Remember that unisexual flowers mean separate male and female flowers.

2. **Defining Autogamy and Geitonogamy**:
- Autogamy refers to self-pollination where a flower fertilizes itself.
- Geitonogamy is a form of self-pollination where pollen from one flower fertilizes another flower on the same plant.

**Hint**: Distinguish between self-pollination (autogamy) and pollination between flowers on the same plant (geitonogamy).

3. **Analyzing Option (a)**:
- The statement claims that autogamy is prevented but not geitonogamy. Since Castor and Maize have unisexual flowers, autogamy is indeed prevented because a flower cannot self-fertilize. However, geitonogamy can occur as pollen can transfer between male and female flowers on the same plant.

**Hint**: Consider how the separation of male and female flowers affects self-pollination.

4. **Analyzing Option (b)**:
- This option states that both autogamy and geitonogamy are prevented. This is incorrect because, while autogamy is prevented, geitonogamy can still occur.

**Hint**: Think about whether pollen can still move between flowers on the same plant.

5. **Analyzing Option (c)**:
- This option suggests that male and female flowers are borne by different plants. While this is true for Castor (dioecious), maize is monoecious, meaning it has both male and female flowers on the same plant. Therefore, this option is not universally applicable.

**Hint**: Check whether the plants in question are dioecious or monoecious.

6. **Analyzing Option (d)**:
- This option states that anthers and stigma are placed at different positions to encourage cross-pollination. While this may be true in some plants, it does not specifically apply to Castor and Maize as a defining characteristic.

**Hint**: Consider whether the positioning of reproductive structures is a key feature in these plants.

### Conclusion:
Based on the analysis, the correct answer is **(a)**: Autogamy is prevented but not geitonogamy.