chitin is the second most abundant organic substance present in the exoskeletion of insects and crustaceans ,It is a

Options

A. Protein
B. polysaccharide and the basic unit is N-Acetyglucosamine
C. protein and `CaCO_(3) ` deposits in it
D. Lipid

Answer

**Step-by-Step Solution:**

1. **Understanding Chitin**: Chitin is identified as a significant organic substance found in the exoskeletons of insects and crustaceans. It is important to recognize its role and composition.

2. **Classification of Chitin**: Chitin is classified as a structural polysaccharide. This means it serves a structural function in biological organisms, particularly in the exoskeletons of arthropods.

3. **Composition of Chitin**: Chitin is a long polymer made up of repeating units. The basic unit of chitin is N-acetylglucosamine, which is a derivative of glucose. This indicates that chitin is composed of amino sugars.

4. **Distinguishing from Other Biomolecules**: It is essential to differentiate chitin from proteins and lipids:
- **Proteins** are made up of amino acids, while chitin is made up of amino sugars.
- **Lipids** are a different class of biomolecules that do not include polysaccharides like chitin.

5. **Conclusion**: Based on the above points, chitin is confirmed to be a structural polysaccharide made up of N-acetylglucosamine units, making it the second most abundant organic substance in the exoskeletons of insects and crustaceans.

**Final Answer**: Chitin is a structural polysaccharide made up of N-acetylglucosamine.

---