Which of the following are present in a dilute aqueous solution of hydrochloric acid?
H₃O⁺ + Cl⁻
H₃O⁺ + OH⁻
Cl⁻ + OH⁻
unionised HCl
Steps: HCl ionises in water producing hydronium ions (H₃O⁺) and chloride ions (Cl⁻). Hence (a).