Hint: Dispersion medium can be defined as the medium where the colloidal particles are dispersed. Colloidal particles are small particles which are suspended in a liquid phase. For example – In a solution of starch, the dispersed phase is starch and water is the dispersion medium.
Complete step by step answer: In smoke, the dispersed phase is in solid state while the dispersion medium is in the gaseous state. In opal, the dispersed phase is in the liquid state whereas the dispersion medium is in the solid state. Opal is a gel. Milk of magnesia is a sol which means that the dispersed phase is in the solid state and the dispersion medium is in the liquid state. Fog is a liquid aerosol. This means that the dispersed phase is in liquid state while the dispersion medium is in gaseous state.
Hence, the correct answer is (A) i.e., gas, solid, liquid, gas respectively.
Note: A student can get confused in between dispersed phase and dispersion medium. Dispersed phase is the phase that is present in the form of colloidal particles. This can be understood by a simple analogy. Consider a solution to be your colloidal system. The dispersion medium in colloids is similar to the solvent in a solution. And the dispersion phase in a colloid is similar to the solute in the solution.