For a reaction in equilibrium, the species involved in the equilibrium constant expression are

The equilibrium constant expression is used to calculate the equilibrium constant of a chemical reaction. The species involved in the expression are those that are in a state of equilibrium, which means they are in constant flux between reactants and products. In order to calculate the equilibrium constant, we need to know the concentrations of the reactants and products in the equilibrium state. Therefore, the species involved in the equilibrium constant expression depend on the state of the reactants and products. Among the options given, the correct answer is the last one: gaseous and dissolved species. This is because when a reaction involves gaseous and dissolved species, their concentrations can change during the reaction, and therefore they need to be included in the equilibrium constant expression. Solid and liquid species, on the other hand, have a fixed concentration during the reaction, and so they are not included in the equilibrium constant expression.