Attainment of "equilibrium state" with the help of "constancy in intensity of colour" is noticed in the case of ... in a closed vessel

Thus, when the color has attained constant intensity, the concentrations of reactants and products have reached equilibrium values. In other words, an equilibrium is attained.

Among the given options, only option-C is correct because iodine gas is colored and easily identified. Whereas, in other options the chemical species are colorless and difficult to identify in terms of color intensity.