Keratin is a ...............protein.

Explanation

**Step-by-Step Solution:**

1. **Understanding Keratin**: Begin by recognizing what keratin is. Keratin is a type of protein found in various structures of animals.

2. **Classification of Keratin**: Keratin can be classified into two main types: soft keratin and hard keratin. This classification helps us understand its different forms and functions.

3. **Nature of Keratin**: Keratin is categorized as a fibrous protein. Fibrous proteins are typically elongated and insoluble in water, providing structural support.

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