The compound that cannot act both as oxidizing and reducing agent is

In ${\mathrm{HNO}}_2$, Nitrogen has oxidation state $+3$, nitrogen has maximum oxidation state $+5$ and minimum $-3$, hence nitrous acid can act both as oxidizing and reducing agent.

Sulphur has maximum oxidation state $+6$ and minimum is $-2$, in ${\mathrm{H}}_2{\mathrm{SO}}_3$ sulphur has oxidation state $+4$, hence it can act as both oxidizing and reducing agent.

For oxygen maximum oxidation state is $+2$ and minimum is $-2$, in ${\mathrm{H}}_2{\mathrm{O}}_2$ Oxygen has oxidation state $-1$, hence it can act bot as oxidizing and reducing agent.

As in ${\mathrm{H}}_3{\mathrm{PO}}_4$, Phosphorous is present in it’s maximum oxidation state i.e., $+5$, hence it cannot act as reducing agent.