The correct answer is option 3: chitin. Chitin is a carbohydrate that is the main component of the exoskeleton or outer skin of most crustaceans. It is a tough and flexible substance that provides structural support and protection for these organisms. Cellulose, option 1, is found in the cell walls of plants and is not a major component of the crustacean exoskeleton. Galactose, option 2, is a monosaccharide commonly found in the form of galactosyl units in many carbohydrates, but it is not the main carbohydrate found in the crustacean exoskeleton. Starch, option 4, is a polysaccharide that serves as a storage form of glucose in plants, and is not a component of the crustacean exoskeleton.