Key Idea Bronsted acid is a substance which has a tendency to donate proton. Bronsted base is a substance which has a tendency to accept proton. HCl can act as Bronsted acid becuase it can only donate proton. HCl+H2O⇌H3O++Cl− The remaining options contains substances which act both as Bronsted acid and Bronsted base. HCO3−+HCO3−NH3+NH3HSO4−+HSO4−⇌H2CO3+CO32−⇌NH4++NH2−⇌H2SO4+SO42− Thus, option (3) is correct.