Explanation:

- Chalk powder: Chalk is primarily made of calcium carbonate. It does not dissolve in water, making it insoluble.

- Sugar: Sugar dissolves easily in water, forming a sweet solution. Therefore, it is soluble.

- Milk: Milk is not a single compound but a colloidal mixture of water, fats, and proteins. It does mix with water, although it's not considered a true solution.

- Salt: Common table salt (sodium chloride) readily dissolves in water, resulting in a saline solution.

Answer: Chalk powder