Answer: Option (a) is the correct answer.
Step-by-step explanation:
Water is a liquid form of matter and in this molecules are less tightly held together due to less intermolecular forces of attraction between them.
As a result, molecules are able to slide past each other.
Whereas in a solid state, molecules are held through strong intermolecular forces of attraction. As a result, they are held in fixed position and they can vibrate in the crystal lattice.
On the other hand, when a substance is in gaseous state then molecules are held through weak Vander waal forces. As a result, molecules move in rapid random motion.
Therefore, we can conclude that the statement they slide around the ions throughout the crystal lattice, best describes the water molecules in a hydrate.