The chitin in the exoskeleton of many arthropods is strengthened by

Analyze the question: It asks what strengthens (hardens) the chitin in the arthropod exoskeleton.

Concept: The arthropod exoskeleton (cuticle) is made mainly of chitin, a tough, flexible polysaccharide (a nitrogen-containing carbohydrate). On its own, chitin is flexible. To make the exoskeleton rigid and strong, it is reinforced in two main ways:

• Proteins (sclerotin) are cross-linked with chitin, a process called sclerotization or tanning. This is common in insects.
• Calcium compounds (calcium carbonate) are deposited in the cuticle, especially in crustaceans like crabs, lobsters and prawns.

Working through the options:

• A. "lids" is probably a misprint of "lipids". Lipids make up a waxy layer that waterproofs the cuticle, but they do not strengthen the chitin.
• B. Proteins do bind with chitin and toughen it. This is partly correct, but they are present in nearly all cuticles as part of the chitin-protein complex.
• C. Calcium compounds: the classic textbook answer for "strengthened by" is calcium salts (calcium carbonate) deposited in the chitin, which hardens it. This is the standard answer for this question.
• D. Organic salt is not how the exoskeleton is hardened.

Note on my working: Option B is also defensible biologically, since proteins do cross-link with chitin.

Answer: C. Calcium compounds

Exam tip: Remember "Chitin + Calcium = Crustacean strength." When a JAMB question asks what hardens the chitin exoskeleton, think calcium carbonate first. Also, watch for typos in options (like "lids" for "lipids"), and recall that lipids give waterproofing, not strength.