Answer : Option B) Pure gas and aqueous solutions.
Explanation : The substances that appear in equilibrium constant expressions are pure gases and aqueous solutions because unlike gases and substances in solution, liquids and solids are observed to have an essentially constant concentration. Which is because liquids and solids are practically incompressible in nature. As the molecules in liquids and solids are placed very close to each other, with very little space to get squeezed and be close to each other. So, as a result, it takes an enormous pressure to perform even a tiny change in volume of the matter. In most of the cases, changes in volume for a solid or liquid substance is ignored.
