Which of the following ions does not liberate hydrogen gas on reaction with dilute acids ?

A compound of molecular formula C₈H₈O₂ reacts with acetophenone to form a single cross-aldol product in the presence of base. The same compound on reaction with conc. NaOH forms benzyl alcohol as one of the products. The structure of the compound is :

The correct sequence of decreasing number of $$\pi $$-bonds in the structures of H₂SO₃, H₂SO₄ and H₂S₂O₇ is :

The incorrect statement among the following is :