Among H₃PO₂,H₃PO₃,H₃PO₄ strongest acid and strongest reducing agent respectively are

As oxidation state decreases, species have more tendency to oxidize then to reduce. So, they have higher reducing nature.

${\mathrm{H}}_3\stackrel{+5}{\mathrm{P}}{\mathrm{O}}_4, {\mathrm{H}}_3\stackrel{+3}{\mathrm{P}}{\mathrm{O}}_3, \mathrm{H}\stackrel{+1}{{ }_3\mathrm{P}}{\mathrm{O}}_2$