Here's why:

* Solid: Barium sulfate forms a crystalline structure with strong ionic bonds between the barium and sulfate ions. These bonds make it rigid and resistant to changes in shape.

* Liquid: Liquids have weaker intermolecular forces and can flow. Barium sulfate does not exhibit these properties.

* Gas: Gases have very weak intermolecular forces and are highly compressible. Barium sulfate does not behave as a gas.

* Aqueous: While barium sulfate is sometimes used in aqueous solutions (like in medical imaging), it doesn't dissolve significantly in water. The term "aqueous" usually implies something is dissolved in water.