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

In ${HNO}_{2}$, 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 ${H}_{2}{SO}_{3}$ 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 ${H}_{2}{O}_{2}$ Oxygen has oxidation state $- 1$, hence it can act bot as oxidizing and reducing agent.

As in ${H}_{3}{PO}_{4}$, Phosphorous is present in it’s maximum oxidation state i.e., $+ 5$, hence it cannot act as reducing agent.