The correct answer is option 1: Aluminium sulphate.

Foam fire extinguishers are a type of fire extinguisher that use a foam or frothy substance to suppress fires. This foam is created by mixing a foaming agent with water. One common foaming agent used in foam fire extinguishers is aluminium sulphate.

Aluminium sulphate is a chemical compound that contains aluminium, sulfur, and oxygen. It is a white, crystalline powder that is soluble in water. When aluminium sulphate is mixed with water, it forms a frothy foam that can help to smother fires by creating a barrier between the fire and the fuel source.

Option 2, copper sulphate, is not correct as it is not commonly used in foam fire extinguishers. Copper sulphate is more commonly used for agricultural purposes, as a fungicide or herbicide.

Option 3, cobalt sulphate, and option 4, nickel sulphate, are also not correct as they are not typically used in foam fire extinguishers. Cobalt sulphate is often used in the production of batteries, pigments, and catalysts. Nickel sulphate is commonly used in electroplating and as a catalyst in chemical reactions.

Therefore, the correct answer is option