- For the reaction at equilibrium, the concentrations of the reactants (\( H_2 \) and \( I_2 \)) decrease, and the concentration of the product (\( HI \)) increases over time.
- As the system reaches equilibrium, the concentration of the reactants will stabilize and the product concentration will also stabilize.
Thus, the graph where the concentrations of \( H_2 \) and \( I_2 \) decrease and the concentration of \( HI \) increases correctly represents the attainment of equilibrium.
Final Answer: Option (3).