Answer:

Washing soda, also known as sodium carbonate decahydrate (Na2CO3.10 H2O), is a hydrate compound. This means that it contains water molecules as part of its crystal structure. When washing soda is exposed to air, it undergoes a process called efflorescence, where the water molecules in the compound are lost to the surrounding atmosphere.

Explanation:

Efflorescence
When a hydrate compound, such as washing soda, is exposed to air, the water molecules present in the compound can evaporate. This process is known as efflorescence. As a result of efflorescence, the compound loses some of its water molecules and its chemical formula changes.

Chemical formula of washing soda
The chemical formula of washing soda, Na2CO3.10 H2O, indicates that it contains 10 water molecules. However, on exposure to air, some of these water molecules are lost.

Changes in the chemical formula
As the water molecules evaporate, the chemical formula of the washing soda changes. The number of water molecules decreases and the chemical formula becomes Na2CO3.x H2O, where 'x' represents the remaining number of water molecules.

Correct answer
In this case, the correct answer is option 'D', Na2CO3.H2O. This implies that on exposure to air, washing soda loses all of its water molecules except for one. Therefore, the chemical formula becomes Na2CO3.H2O.

Conclusion
When washing soda (Na2CO3.10 H2O) is exposed to air, it undergoes efflorescence and loses some of its water molecules. The correct chemical formula for washing soda after exposure to air is Na2CO3.H2O.