Which of the following macromolecules is a protein ?

Options

A. Glycogen
B. Keratin
C. Cholesterol
D. Chitin

Explanation

To determine which of the following macromolecules is a protein, we need to analyze each option provided:

1. **Glycogen**:
- Glycogen is a multi-branched polysaccharide. It serves as a form of energy storage in humans, animals, fungi, and bacteria. Since glycogen is a carbohydrate, it is not a protein.

2. **Keratin**:
- Keratin is a fibrous structural protein. It is a key structural material that forms hair, horns, and the outer layer of human skin. Since keratin is classified as a protein, it is a potential answer.

3. **Cholesterol**:
- Cholesterol is a fat-like waxy substance that plays a role in synthesizing hormones and vitamin D. Cholesterol is a type of lipid, not a protein.

4. **Chitin**:
- Chitin is a fibrous substance consisting of polysaccharides. It is the major constituent of the exoskeleton of arthropods and the cell wall of fungi. Since chitin is a polysaccharide, it is also not a protein.

After analyzing all the options, the only macromolecule that is a protein is **keratin**.

### Final Answer:
The macromolecule that is a protein is **keratin**.