The caesin of milk is a

Options

A. Lipid
B. Carbohydrate
C. Protein
D. Steroid

Answer

### Step-by-Step Solution:

1. **Understanding Casein**: Casein is a type of protein found in milk. It is one of the primary proteins present in dairy products.

2. **Composition in Milk**: In cow's milk, casein constitutes about 80% of the total protein content. In human milk, it varies between 20% to 60%.

3. **Amino Acids**: Casein is primarily made up of amino acids, with a high concentration of proline amino acids. This specific composition affects its solubility.

4. **Solubility**: Due to the high amount of proline, casein is less soluble in water, making it hydrophobic in nature.

5. **Identifying the Correct Option**: Given the options:
- Lipid: Incorrect, as casein is not a fat.
- Carbohydrate: Incorrect, as casein is not a sugar.
- Protein: Correct, as casein is a type of protein.
- Steroid: Incorrect, as steroids are derived from lipids.

6. **Conclusion**: The correct answer is that casein of milk is a protein.