When HCl is passed through water then being a polar covalent compound it ionises as:
HCl (aq)→H++Cl−
It forms hydronium ion in the solution due to the combination of hydrogen ion with water molecule. The reaction takes place as follows:
H++H2O→H3O+
So, dilution of HCl will form hydronium ion and chloride ion. Hence, option (a) is correct.