When water was added to a white anhydrous substance X, the colour changed to blue. X is

The white anhydrous substance X changes color to blue when water is added. This is an indication of a chemical reaction taking place, and the resulting blue color is likely due to the formation of a complex compound. Out of the given options, CuSO4 is a blue compound, also known as copper(II) sulfate, and is commonly used as a colorant and in various industrial applications. When it is dissolved in water, it forms a blue solution due to the hydration of copper ions. Therefore, it is most likely that the anhydrous substance X is CuSO4 because the addition of water resulted in a blue color, which is consistent with the characteristic blue color of CuSO4.