Which development is marked by the beginning of adolescence?

Options

A. Development of cells
B. Development of heart tissues
C. Development of nails and hair
D. Development of reproductive organs

Answer

**Step-by-Step Solution:**

1. **Understanding Adolescence**: Adolescence is a developmental stage that occurs between childhood and adulthood. It is characterized by various physical, emotional, and psychological changes.

2. **Identifying Key Developments**: The question asks which specific development marks the beginning of adolescence. We need to evaluate the given options to find the correct one.

3. **Evaluating the Options**:
- **Option 1: Development of Cells**: This option refers to the general growth of cells, which is not specific to adolescence. This is incorrect.
- **Option 2: Development of Heart Tissues**: This development occurs during embryonic stages, specifically in the early weeks of pregnancy. It does not relate to adolescence. This is incorrect.
- **Option 3: Development of Nail or Hair**: While hair and nails do grow during adolescence, their development does not specifically mark the beginning of this stage. This is incorrect.
- **Option 4: Development of the Reproductive Organs**: This development is crucial as it signifies the onset of puberty, which is a key feature of adolescence. This is the correct answer.

4. **Conclusion**: The development that marks the beginning of adolescence is the development of the reproductive organs.

**Final Answer**: Development of the Reproductive Organs.

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