Which of the following must be included in a Kc expression?

In a Kc expression, only the concentrations of gases and aqueous solutions are included. This is because the equilibrium constant, Kc, is defined in terms of the concentrations of reactants and products that are present in the gaseous state or in solution.

Solids and liquids do not appear in the equilibrium expression because their concentrations remain constant throughout the reaction. When calculating Kc, the activity (which can be thought of as the effective concentration) of pure solids and liquids is taken to be one. Therefore, they do not affect the equilibrium position, and their contributions are omitted from the Kc expression.

Thus, only the species that can change concentration during the chemical equilibrium, specifically gases and substances dissolved in a solution, are included in Kc calculations. This is why the answer indicating gases and aqueous solutions is the correct choice.