Answer is: HCl (hydrochloric acid).
Hydrochloric acid is strong acid and in water dissociates:
HCl(aq) → H⁺(aq) + Cl⁻(aq).
Because acid gives a lot of hydrogen cations (H⁺), pH (pH = -log[H⁺]) is lower than seven (acidic solution).
Ammonia (NH₃) and barium hydroxide (Ba(OH)₂) are base and their pH is higher that seven. Sodium fluoride (NaF) is neutral salt.
