Answer is: hydrogen ion concentration (H⁺) is greater in hydrochloric acid than a lemon with same concentration of both solutions.
Strong acids (in this example hydrochloric acid) completely ionize in water and give large amount of hydrogen ions (H⁺): HCl(aq) → H⁺(aq) + Cl⁻(aq).
Weak acids (lemon has citric acid) partially ionize in water and give only a few hydrogen ions (H⁺), in the solution there molecules of acid and ions.