Hydrogen chloride (HCl) is the gas commonly used in the fountain experiment because of its exceptionally high solubility in water. When a small amount of water is introduced into a flask filled with HCl gas, the gas dissolves rapidly, creating a partial vacuum.

B. hydrogen sulphide (H\(_2\)S) is not typically used for the fountain experiment. While it has some solubility, it is not as highly soluble as HCl or NH\(_3\), which are necessary for the dramatic effect of the fountain. It is also a toxic gas with a foul odor.

C. dinitrogen(I) oxide (nitrous oxide, N\(_2\)O) and D. nitrogen(II) oxide (nitric oxide, NO) are gases with low solubility in water compared to HCl, and would not produce the significant pressure drop required for a successful fountain experiment.

Therefore, the correct answer is option A.