Step 1: Understanding the process.
Sodium hydroxide (NaOH) is commonly used in the manufacture of soft soap. The reaction involves the saponification of fats and oils with sodium hydroxide, producing soap and glycerol.
Step 2: Analyzing the options.
(A) Lithium: Incorrect. Lithium hydroxide is not commonly used in soap making.
(B) Caesium: Incorrect. Caesium hydroxide is highly reactive and not used in soap making.
(C) Potassium: Incorrect. Potassium hydroxide is used in soft soap, but sodium is more common.
(D) Sodium: Correct. Sodium hydroxide is the most common alkali used to make soft soap.
Step 3: Conclusion.
The correct answer is (D) Sodium, as sodium hydroxide is widely used in the production of soft soap.