Which of the following oxoacids of phosphorus has strongest...

$\mathrm{P}-\mathrm{H}$ bonds found in phosphorus acids have reducing properties. Thus, reducing property is directly proportional to number of $\mathrm{P}-\mathrm{H}$ bonds. The structures of given oxoacids of phosphorus are as follows:
$\mathrm{H}_3 \mathrm{PO}_2$ contains $2 \mathrm{P}-\mathrm{H}$ bonds which is maximum among given options. Thus, $\mathrm{H}_3 \mathrm{PO}_2$ has strongest reducing property.