The correct statement(s) related to oxoacids of phosphorous is(are)

$4{\mathrm{H}}_3{\mathrm{PO}}_3\stackrel{∆}{\to }$$3{\mathrm{H}}_3{\mathrm{PO}}_4+{\mathrm{PH}}_3$ (Disproportionation)

Oxidation state of $\text{'}\mathrm{P}\text{'}$ in ${\mathrm{H}}_3{\mathrm{PO}}_4$ is $+5$ which is maximum oxidation state of $\text{'}\mathrm{P}\text{'}$ so ${\mathrm{H}}_3{\mathrm{PO}}_4$ cannot act as a reducing agent whereas ${\mathrm{H}}_3{\mathrm{PO}}_3$ can act as a reducing agent because the oxidation state is $+3$ and it can increase upto $+5$.

$\Rightarrow$ ${\mathrm{H}}_3{\mathrm{PO}}_3$: 

It is dibasic acid.

$\Rightarrow$ Only $\mathrm{H}$- atom attached with $\text{'}\mathrm{O}\text{'}$ is ionizable.