Which of the following is a tetrabasic acid?

Tetrabasic acid means acids having $4-$ replacable hydrogen atoms.

$H_{3}P{O}_2⟶$ hypophosphorus acid

Metaphosphoric acid$⟶(HP{O}_3{)}_n$

Pyrophosphoric acid $⟶H_4{P}_2{O}_7$

Orthophosphoric acid $⟶H_{3}P{O}_4$

Here, pyrophosphoric acid, $H_4{P}_2{O}_7$ have $4$ removable $H^{+}$ ions, hence is a tetrabasic.