Insulin is

Options

A. An amino acid
B. Protein
C. A carbohydrate
D. A lipid

Explanation

**Step-by-Step Solution:**

1. **Identify the Nature of Insulin**: The question asks about the classification of insulin. We need to determine whether it is an amino acid, protein, carbohydrate, or lipid.

2. **Composition of Insulin**: Insulin is made up of 51 amino acids. This is a crucial piece of information as it indicates that insulin is a polypeptide.

3. **Structure of Insulin**: Insulin consists of two chains: the alpha chain and the beta chain. The alpha chain contains 21 amino acids, while the beta chain contains 30 amino acids. This structural information further confirms that insulin is a protein.

4. **Function of Insulin**: Insulin is secreted by the pancreas and plays a vital role in regulating blood sugar levels. This function is important in understanding the biological significance of insulin.

5. **Conclusion**: Based on its composition and structure, insulin is classified as a protein.

**Final Answer**: Insulin is a protein.

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