Answer: D) a gas
Step-by-step explanation:
Equilibrium state is the state when reactants and products are present but the concentrations does not change with time.
A chemical equilibrium is dynamic in nature forward and backward reactions continue for indefinite time and never stops. Chemical equilibrium is attained in closed system i.e the species present in the system cannot leave.
Thus if the product formed is a gas , it will escape and thus equilibrium will be disturbed.
Thus reaction does not reach a state of dynamic equilibrium because one of the products formed is gas.