Which of the following oxoacids of phosphorus has strongest reducing property?

Hypophosphorous acid is a good reducing agent as it contains two P–H bonds.

$4{\mathrm{AgNO}}_3+2{\mathrm{H}}_2\mathrm{O}+{\mathrm{H}}_3{\mathrm{PO}}_2\underset{ }{\overset{ }{\to }}4\mathrm{Ag}+4{\mathrm{HNO}}_3+{\mathrm{H}}_3{\mathrm{PO}}_4$

Reducing property $\propto$number of P-H bonds