The correct answer is option 3: Sodium stearate is used to make soap.

To understand why option 3 is the correct answer, let`s briefly look at the other options:

Option 1: Sodium stearate is not used in gunpowder. Gunpowder is typically made up of sulfur, charcoal, and potassium nitrate.

Option 2: Sodium stearate is not used in paint. Paint is made up of pigments, binders, solvents, and additives, but sodium stearate is not one of the common ingredients in paint.

Option 4: Sodium stearate is not used to make fertilizer. Fertilizers are commonly made up of various nutrients, such as nitrogen, phosphorus, and potassium, to promote plant growth. Sodium stearate does not serve a purpose in fertilizer production.

Therefore, the correct answer is option 3: Sodium stearate is a salt that is commonly used to make soap. Sodium stearate is a key ingredient in soap production as it acts as a surfactant, helping to remove dirt and grease from surfaces. It is commonly derived from animal fats or vegetable oils, such as coconut oil or palm oil, and contributes to the cleansing properties of soap.