Crystals of washing soda kept in dry air gradually become a white powder. This change is

Adeliquescence, giving $NaOH$
Bdecomposition to $Na X_{2} O$ and $CO X_{2}$
Cefflorescence, giving a lower hydrate such as $Na X_{2} CO X_{3} \cdot H X_{2} O$
Doxidation to $Na X_{2} CO X_{4}$

Answer: C

Na X_{2} CO X_{3} \cdot 10 H X_{2} O

has a water vapour pressure above that of dry air, so it loses water:

Na X_{2} CO X_{3} \cdot 10 H X_{2} O Na X_{2} CO X_{3} \cdot H X_{2} O + 9 H X_{2} O

. Loss of water of crystallisation in air is called efflorescence.