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

Step 1 — For a compound to act as both oxidising and reducing agent, the central atom must be in an intermediate oxidation state.

Step 2 — Check each compound:

| Compound | Element | Oxidation State | Can be oxidised? | Can be reduced? | |---|---|---|---|---| | $\mathrm{H_3PO_4}$ | P | $+5$ (maximum) | No (already max) | Yes | | $\mathrm{HNO_2}$ | N | $+3$ (intermediate) | Yes → $+5$ | Yes → $-3$ | | $\mathrm{H_2SO_3}$ | S | $+4$ (intermediate) | Yes → $+6$ | Yes → $-2$ | | $\mathrm{H_2O_2}$ | O | $-1$ (intermediate) | Yes → $0$ | Yes → $-2$ |

Step 3 — Conclusion: $\mathrm{H_3PO_4}$: P is at its maximum oxidation state (+5). It cannot be further oxidised, so it cannot act as a reducing agent. It can only act as an oxidising agent.

$\mathrm{HNO_2}$, $\mathrm{H_2SO_3}$, and $\mathrm{H_2O_2}$ all have elements in intermediate oxidation states → can act as both.

Answer: Option (1) — $\mathrm{H_3PO_4}$