Answer : The volume of water produced by this reaction would be, 4.75 liters.
Explanation : Given,
Volume of HCl gas = 9.5 L
The balanced chemical reaction will be:

As we know that at STP, 1 mole of gas occupy 22.4 L volume of gas.
By the stoichiometry we can say that:
As, 4 liter volume of HCl gas react to give 2 liter volume of water
So, 9.5 liter volume of HCl gas react to give
volume of water
Therefore, the volume of water produced by this reaction would be, 4.75 liters.