(c) Hydrogen chloride gas and water
When HCl gas dissolves in water, it ionises to form H⁺ ions, which combine with water molecules to form hydronium ions: HCl + H₂O → H₃O⁺ + Cl⁻.
Hydronium ion (H₃O⁺) is formed when H⁺ (from an acid) combines with a water molecule. HCl is an acid that dissociates in water to release H⁺, which then bonds with H₂O to give H₃O⁺. NaOH gives OH⁻ (not H₃O⁺), CaCl₂ is a salt with no such reaction, and ethanol does not ionise in water.