The correct answer is indeed 3, which corresponds to 5 water molecules.

To better understand, we have to delve into the concept of hydration, a type of chemical reaction where water molecules associate with ions. Some compounds, like copper sulphate (CuSO4), form crystals that incorporate water molecules into their structure. This phenomenon is known as water of crystallization.

Specifically, copper sulphate forms a pentahydrate (CuSO4.5H2O), meaning it combines with 5 water molecules to form a crystalline structure.

Option 1 and 2 are incorrect as the number of water molecules is not 2 or 4. Option 4 is also incorrect because the molecule is not associated with 6 water molecules. The copper sulphate pentahydrate clearly states that the number of water molecules is 5, validating Option 3 as the correct answer.