An example of a hygroscopic substance is

Correct answer: CaCl₂(s)

Explanation: A hygroscopic substance absorbs moisture from the air. Calcium chloride is strongly hygroscopic and is widely used as a drying agent (desiccant) in laboratories and industry.

• CuO(s) — Incorrect. Copper(II) oxide is not hygroscopic.
• MgCl₂(s) — Absorbs moisture and can be deliquescent, but the standard JAMB example of a hygroscopic drying agent is CaCl₂.
• CaCl₂(s) — Correct. Classic hygroscopic desiccant.
• NaOH(s) — Also absorbs moisture (and CO₂) and is deliquescent, but CaCl₂ is the expected answer.