Answer is: d. a higher concentration of unionized acid molecules than a stronger acid in water.
For example:
Chemical reaction 1: HCl(aq) + H₂O(l) → H₃O⁺(aq) + Cl⁻(aq).
Chemical reaction 2: CH₃COOH(aq) + H₂O(l) ⇄ H₃O⁺(aq) + CH₃COO⁻(aq).
Hydrochloric acid (HCl) is strong acid, which means that concencentration of hydronium ions are equal as concentration of hydrochloric acid.
Acetic acid (CH₃COOH) is weak acid and does not dissociate completely as HCl, so concentration of hydronium ion in this solutions are very low.
