In a disproportionation reaction, the same element undergoes both oxidation and reduction simultaneously. In H₂O₂ decomposition, oxygen is in the −1 oxidation state in H₂O₂. In the products, oxygen in H₂O is −2 (reduction) and oxygen in O₂ is 0 (oxidation). Thus oxygen in H₂O₂ is simultaneously oxidised and reduced — the hallmark of disproportionation. Option B is a displacement reaction, option C is a combination reaction, and option D is a reduction of iron oxide by CO (a redox reaction but not disproportionation, since different elements are oxidised and reduced).