JEE Main · 2021 · 16 Mar 2021 · Shift 1 · p-Block Elements

The oxidation state of phosphorus follows from the formula once hydrogen is taken as $+1$ and oxygen as $-2$.
Hypophosphorous acid is ${\mathrm{H}}_3\mathrm{P}{\mathrm{O}}_2$, which gives $+1$.
Orthophosphoric acid is ${\mathrm{H}}_3\mathrm{P}{\mathrm{O}}_4$, which gives $+5$.
Hypophosphoric acid is ${\mathrm{H}}_4{\mathrm{P}}_2{\mathrm{O}}_6$, with a P-P bond, and each phosphorus is $+4$.
Orthophosphorous acid is ${\mathrm{H}}_3\mathrm{P}{\mathrm{O}}_3$, which gives $+3$.
So hypophosphorous acid pairs with $+1$, orthophosphoric acid with $+5$, hypophosphoric acid with $+4$ and orthophosphorous acid with $+3$.