Keratin, a structural protein is present in:

Options

A. Hair
B. Wool
C. Silk
D. All of these

Explanation

### Step-by-Step Solution:

1. **Understanding Keratin**: Keratin is a type of structural protein that is primarily found in various parts of the body. It is known for its fibrous nature and plays a crucial role in providing strength and protection.

2. **Identifying the Locations of Keratin**: Keratin is predominantly present in:
- **Hair**: Keratin forms the main structural component of hair, providing it with strength and resilience.
- **Skin**: In the skin, keratin helps in forming a protective barrier against environmental damage.
- **Nails**: Keratin is also a key component of nails, contributing to their hardness and durability.
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