Since there are 10 water molecules in washing soda, hence it is known as 

Sodium carbonate is the name for washing soda. Washing soda is called sodium carbonate decahydrate because it contains 10 water molecules. The powder form of washing soda is white and flavorless. It is a hygroscopic substance, which is a substance that absorbs moisture. It may create potent alkaline water solutions and is very soluble in water. The powder form of washing soda is white and flavourless. It is referred to as sodium carbonate and has the chemical formula Na₂CO₃. It is a hygroscopic material, which is a substance that absorbs moisture. It may create potent alkaline water solutions and is very soluble in water.Making washing soda is a simple technique that Sodium chloride, ammonia, and carbon dioxide are all created by a reaction in Solvay's method, and the leftover calcium oxide is used to separate ammonia from ammonium chloride.
When heated, sodium bicarbonate, which is produced initially, turns into sodium carbonate:

$\mathrm{NaCl}+{\mathrm{NH}}_3+{\mathrm{CO}}_2+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{NaHCO}}_3+{\mathrm{NH}}_4\mathrm{Cl}$
$2{\mathrm{NaHCO}}_3\to {\mathrm{Na}}_2{\mathrm{CO}}_3+{\mathrm{H}}_2\mathrm{O}+{\mathrm{CO}}_2$
${\mathrm{Na}}_2{\mathrm{CO}}_3+10{\mathrm{H}}_2\mathrm{O}\to {\mathrm{Na}}_2{\mathrm{CO}}_3·10{\mathrm{H}}_2\mathrm{O}$

The washing soda is then created. As you can see, sodium bicarbonate has 10 molecules of crystallising water added to it. It is referred to as sodium bicarbonate decahydrate as a result.

So, the correct answer is “Option B”.