Final answer:
Water molecules are closest together in the solid phase. This is due to the particular structure of ice. The molecules in liquids and gases have more space between them.
Step-by-step explanation:
The phase in which the water molecules are closest together is solid (option C). In a solid state, molecules are closely packed together, often in a regular pattern. This is contrasted with a liquid state where molecules are close but can move past each other, and a gaseous state where molecules spread out to fill their container. Water, unlike many other substances, is more dense as a liquid than a solid, but molecules are still closer together in its solid form because of the particular structure of ice. Dry ice is solid carbon dioxide, not water, and is therefore not a valid choice.
Learn more about States of water