Which of the following oxoacid sulfur is known not to bear any acid salt?

Dithionic acid

Dithionic acid is formulated as H₂S₂O₆ and is known to possess no acid salts i.e. salt formed due to the loss of one hydrogen ion. The diprotic salts of dithionic acid is known as dithionates which are mild oxidizing and reducing agents as well.