The compound that can work both as an oxidising as well as a reducing agent is;

The correct answer is H₂O₂ (Hydrogen Peroxide).

Step 1: Understanding the Key Concept

For a compound to act as both an oxidising agent and a reducing agent, the element in that compound must be in an intermediate oxidation state. This means:
- It can gain electrons (get reduced) → acts as an oxidising agent
- It can lose electrons (get oxidised) → acts as a reducing agent

Step 2: Determining the Oxidation State of Oxygen in H₂O₂

In hydrogen peroxide (H₂O₂), the oxidation state of oxygen is −1.

This is an intermediate oxidation state for oxygen, which can range from:
- 0 (in O₂, its elemental form) — the highest common state
- −2 (in H₂O, most oxides) — the lowest common state

Since oxygen in H₂O₂ is at −1 (between 0 and −2), it can move in either direction.

Step 3: H₂O₂ as an Oxidising Agent

When H₂O₂ acts as an oxidising agent, the oxygen in H₂O₂ gets reduced from −1 to −2.

Example: With ferrous ions (Fe²⁺):

$H_2{O}_2+2{\mathrm{Fe}}^{2+}\to 2{\mathrm{Fe}}^{3+}+2{\mathrm{OH}}^{-}$

Here, oxygen goes from −1 → −2 (reduced), so H₂O₂ is the oxidising agent.

Step 4: H₂O₂ as a Reducing Agent

When H₂O₂ acts as a reducing agent, the oxygen in H₂O₂ gets oxidised from −1 to 0.

Example: With acidified KMnO₄:

$5H_2{O}_2+2{\mathrm{KMnO}}_4+3H_2{\mathrm{SO}}_4\to 5O_2+2{\mathrm{MnSO}}_4+K_2{\mathrm{SO}}_4+8H_{2}O$

Here, oxygen goes from −1 → 0 (oxidised), so H₂O₂ is the reducing agent.

Step 5: Why Not the Other Options?

• KMnO₄ — Mn is in its highest oxidation state (+7). It can only be reduced, so it acts only as an oxidising agent.
• HNO₃ — Nitrogen is in the oxidation state of +5 (near its maximum). It predominantly acts as a strong oxidising agent and does not function as a reducing agent under normal conditions.
• K₂Cr₂O₇ — Cr is in its highest oxidation state (+6). It can only be reduced, so it acts only as an oxidising agent.

Conclusion: Among all the given options, only H₂O₂ contains oxygen in an intermediate oxidation state (−1), enabling it to act as both an oxidising agent (−1 → −2) and a reducing agent (−1 → 0). This dual behaviour is known as disproportionation capability.