Groundnut belongs to the family Fabaceae. And the characteristic fruit of this family is Legume. Hence the fruit found in groundnut is legume.
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Reference- NCERT. Morphology of flowering plants

Answer:

Introduction:
Groundnut, also known as peanut, is a popular legume crop that is widely cultivated for its seeds. The fruit of the groundnut plant is a legume rather than a nut, making option 'A' the correct answer.

Explanation:
Groundnut plants belong to the family Fabaceae, which is known for its leguminous plants. The fruit of the groundnut plant is a pod that develops underground, containing the seeds or kernels. Let's understand why the fruit of groundnut is considered a legume rather than a nut:

1. Definition of a Legume:
- Legumes are a type of fruit that develops from a simple ovary and typically splits open along two seams to release the seeds.
- Legumes are characterized by their ability to fix atmospheric nitrogen through a symbiotic relationship with nitrogen-fixing bacteria called rhizobia.

2. Fruit Structure of Groundnut:
- The fruit of groundnut is a pod or legume that contains 1 to 4 seeds.
- The pod is formed from the ovary of the flower and has a fleshy, leathery texture.
- It develops underground, making it a unique characteristic of the groundnut plant.
- The pod has two seams, and when it matures, it splits open to reveal the seeds.

3. Comparison with Other Fruits:
- A caryopsis, commonly known as a grain, is a type of fruit where the seed coat is fused with the ovary wall, making it inseparable.
- A berry is a fleshy fruit that typically develops from a single ovary and contains multiple seeds.
- A nut is a hard-shelled fruit that does not split open to release the seed.
- In the case of groundnut, it does not meet the characteristics of a caryopsis, berry, or nut. Instead, it meets the criteria of a legume due to its pod structure and seed release mechanism.

Conclusion:
The fruit of the groundnut plant is a legume. It develops as a pod underground and has two seams that split open to release the seeds. Understanding the botanical classification of groundnut helps in recognizing its unique characteristics and differentiating it from other types of fruits.