Option 1: Magnesium sulphate is not used in making gunpowder. Magnesium sulphate is commonly known as Epsom salt and is used for various purposes such as relieving muscle aches and as a fertilizer.
Option 2: Potassium nitrate is the correct answer. This compound, also known as saltpeter, is a key ingredient in gunpowder. It provides the oxygen necessary for the combustion reaction that propels the bullet or projectile.
Option 3: Sodium stearate is not used in making gunpowder. Sodium stearate is a common ingredient in soap and is used as a lubricant and emulsifier.
Option 4: Calcium sulphate is not used in making gunpowder. Calcium sulphate is commonly known as gypsum and is used in building materials and as a food additive.
Therefore, the correct answer is option 2, Potassium nitrate.