S-I : ${\text{H}}_{\text{3}}{\text{PO}}_{\text{2}}$ is a stronger acid than ${\text{H}}_{\text{3}}{\text{PO}}_{\text{4}}$ under identical conditions.

S-II : The oxidation number of phosphorus in ${\text{H}}_{\text{3}}{\text{PO}}_{\text{2}}$ is +1, where as in ${\text{H}}_{\text{3}}{\text{PO}}_{\text{4}}$ it is +5.

Then, the correct statements are         

In ${\text{H}}_{\text{3}}{\text{PO}}_{\text{2}}$, there is one -OH group whereas in ${\text{H}}_{\text{3}}{\text{PO}}_{\text{4}}$, there are 3 -OH groups. More is the -OH groups, lesser is the acidic strength. This means that S-I is correct.

Oxidation number of P in ${\text{H}}_{\text{3}}{\text{PO}}_{\text{2}}$ is +3 and in ${\text{H}}_{\text{3}}{\text{PO}}_{\text{4}}$, oxidation state of P is +5. This means that S-II is correct.