The correct option is
D All the above
Chitin is a polysaccharide. It is an important component of a cell wall of fungi. It is the main constituent of the exoskeleton of the arthropods. It is a long chain polymer of N-acetylglucosamine. It is the second most abundant homopolysaccharide. Crab and prawns have chitin in the exoskeleton while Agaricus has chitin in the cell wall.
So, the correct answer is option D.