Insulin is a

Options

A. sugar
B. lipid
C. vitamin
D. hormone

Explanation

**Step-by-Step Solution:**

1. **Identify the Options**: The question presents four options: sugar, lipid, vitamin, and hormone. We need to determine which category insulin belongs to.

2. **Understand What Insulin Is**: Insulin is a substance produced by the body. It is known to regulate glucose levels in the blood.

3. **Define Hormones**: Hormones are chemical substances that act as messengers in the body, affecting various physiological processes. They are produced in one part of the body and exert their effects on different target organs or tissues.

4. **Locate the Source of Insulin**: Insulin is secreted by the beta cells of the islets of Langerhans in the pancreas. This is crucial for its classification as a hormone.

5. **Analyze the Structure of Insulin**: Insulin is a protein composed of two peptide chains (Chain A and Chain B). Chain A consists of 21 amino acids, while Chain B contains 30 amino acids. These chains are linked by disulfide bonds, which is characteristic of proteins.

6. **Eliminate Other Options**:
- **Sugar**: Insulin is not a sugar because sugars are carbohydrates, while insulin is a protein.
- **Lipid**: Insulin is not a lipid; lipids are primarily composed of hydrocarbons and serve as structural components or energy storage.
- **Vitamin**: Insulin is not a vitamin, as vitamins are organic compounds required in trace amounts for various bodily functions.

7. **Conclusion**: Based on the above analysis, insulin is classified as a hormone.

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

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