C. If you double the volume of the vessel, you decrease the pressure in the system. Hence, the system will favour the reaction that increases the pressure. However, on both sides of the reaction (H2 + Cl <-> 2HCl) there are two molecules: one H2 and one Cl molecule before and two HCl molecules afterwards. Therefore, the system cannot increase the pressure by favouring a reaction: if it favours the forward reaction, the pressure remains the same as if it favours the reverse reaction. Thus, there is no overall effect.
(If H2+Cl created a single molecule, the reverse reaction would be favoured so as to increase the pressure. If H2+Cl created three molecules, the forward reaction would be favoured to increase the pressure)