Stem is modified into the flat green and succulent structure in

Options

A. Casurina
B. Hydrilla
C. Acacia
D. Opuntia

Answer

### Step-by-Step Solution:

1. **Understanding the Question**: The question asks about a plant where the stem is modified into a flat, green, and succulent structure. This indicates that we are looking for a specific type of stem modification.

2. **Identifying the Type of Modification**: The modification of the stem into a flat, green, and succulent structure is known as a "phylloclade." Phylloclades are adaptations that allow plants to survive in arid conditions by taking over the function of photosynthesis.

3. **Recognizing the Function of Phylloclades**: In plants with phylloclades, the leaves are often reduced to spines to minimize water loss through transpiration. The stem then becomes the primary site for photosynthesis.

4. **Identifying the Correct Plant**: The question provides options, and we need to identify which plant exhibits this modification. The plant "Apuncia" (commonly known as Opuntia or prickly pear cactus) is known for having phylloclades.

5. **Analyzing Other Options**:
- **Hydrilla**: An aquatic plant, which does not have phylloclades.
- **Acacia**: A terrestrial plant that does not exhibit this modification.
- **Euphorbia**: Another plant that can have similar adaptations, but the question specifically asks for the one that is most recognized for this structure.

6. **Conclusion**: Based on the understanding of phylloclades and the characteristics of the plants listed, the correct answer is **Apuncia**.

### Final Answer:
The stem is modified into a flat green and succulent structure in **Apuncia**.

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