A compound that is a white solid which absorbs water vapour from the air is :

Calcium chloride (CaCl2) is a white solid compound known for its strong hygroscopic and deliquescent properties. Hygroscopy is the phenomenon where a substance attracts and holds water molecules from the surrounding environment [2]. Calcium chloride is particularly effective at this, often used as a desiccant because it can absorb significant amounts of water vapour from the air. When exposed to high humidity, it undergoes deliquescence, meaning it absorbs enough moisture to eventually dissolve and form a liquid solution. In contrast, sodium carbonate is typically efflorescent (losing water to the air), and calcium sulphate (as gypsum or plaster of Paris) does not exhibit the same rapid moisture absorption leading to dissolution [1]. Therefore, calcium chloride is the most suitable answer for a white solid that absorbs water vapour from the air.

Sources

1. [2] Science , class X (NCERT 2025 ed.) > Chapter 4: Carbon and its Compounds > Activity 4.9 > p. 74
2. [1] https://en.wikipedia.org/wiki/Hygroscopy