Which of the following oxo acids of phosphorus is a reducing agent?

An oxyacid, oxoacid, or ternary acid is an acid that contains oxygen. Specifically, it is a compound that contains hydrogen, oxygen, and at least one other element, with at least one hydrogen atom bonded to oxygen that can dissociate to produce the H⁺ cation and the anion of the acid.

Oxoacids of Phosphorus are Hypophosphoric acid(H₃PO₄), Metaphosphoric acid(HPO₂), Pyrophosphoric acid (H₄P₂O₇), Hypophosphorous acid(H₃PO₂), Phosphorous acid (H₃PO₃), Peroxophosphoric acid (H₃PO₅), Orthophosphoric acid (H₃PO₅). Oxoacids are acids containing oxygen.

Hypophosphorus acid (H3​PO2​) is a monobasic acid which act as reducing agent. In this molecule two P−H bonds are responsible for its reducing character and one O−H bond is responsible for its monobasic acid character

${\mathrm{H}}_3{\mathrm{PO}}_3$is $\mathrm{HOP}(\mathrm{OH}{)}_2\text{.}$The bond $\mathrm{P}-\mathrm{H}$ is responsible for reducing agent.