Which of these gases is highly soluble in water?

HCl

The solubility of a gas can be demonstrated by means of Fountain Experiment. Take a dry round bottomed flask filled with dry HCl gas. On the mouth of the flask, fix a rubber stopper with two holes. Through one hole pass along the jet tube, and through other hole pass a dropper with few drops of water. Put the jet in the beaker containing blue litmus solution. Press the dropper. It is seen that the blue litmus solution enters the jet tube with a great force, forming a red fountain.

As the water goes in the flask from the dropper, HCl gas present in the flask dissolves due to its high solubility, thereby lowering the pressure inside.