Which of these is not a protein?

A.Correct option D

B.Explanation for correct option:

• N-acetylglucosamine is a monosaccharide and derivative of glucose. 
• It is an amide between glucosamine and acetic acid. It has a molecular formula of C${}_8$H${}_{15}$NO${}_6$.
• Chitin is a polysaccharide.
• It is an important component of the cell wall of fungi.
• It is a long-chain polymer of N-acetylglucosamine. It is a modified form of glucose.
• The bond formed between the glucose molecule is known as glycosidic linkage.