The correct answer is: B. By decreasing solubility of Soap.
Saponification is a chemical reaction that produces soap and glycerin from fats or oils and an alkali. The alkali can be sodium hydroxide (NaOH), potassium hydroxide (KOH), or another strong base.
When soap is made, the fats or oils are first heated until they melt. Then, the alkali is added and the mixture is stirred. This causes a chemical reaction to occur, which produces soap and glycerin. The soap is then separated from the glycerin and water by filtration.
Common salt (NaCl) can be added to the soap solution after saponification. This helps to decrease the solubility of the soap, which makes it easier to separate from the solution. The salt ions interact with the soap molecules, which makes them less likely to dissolve in water. This results in a precipitate of soap, which can be easily filtered out of the solution.
The other options are incorrect. Option A is incorrect because common salt does not decrease the density of soap. Option C is incorrect because common salt does not increase the density of soap. Option D is incorrect because common salt does not increase the solubility of soap.