If $\text{X}$ = $2{\text{CuSO}}_4.10{\text{H}}_2\text{O}$. How many water molecules are present in one formulae unit of X?

The correct option is D 5

Water of crystalization is the fixed number of water molecules in one formulae unit of a salt.X. Here 2 formula unit of salt X has 10 water molecules. Therefore, number of water molecules in one formula unit of salt $=\frac{10}{2}=5$.
Hence, number of water molecules present in one formula unit of salt X is 5.