Which substance does not exhibit disproportionation reaction?

Disproportionation reactions occur when a single substance is simultaneously oxidized and reduced, resulting in the formation of two different products from that substance. To determine which of the listed substances does not undergo disproportionation, we can analyze the redox behavior of each one.

Chlorine monoxide (ClO) does not undergo disproportionation because it does not have multiple oxidation states available for the chlorine atom to transition between during a reaction. In contrast, the other substances listed have the capacity to both lose and gain electrons.

Chlorine dioxide (ClO₂) can disproportionate because chlorine in this compound can exist in an oxidation state that allows it to be both oxidized and reduced. Similarly, hydrogen peroxide (H₂O₂) is well-known for undergoing disproportionation reactions, often breaking down into water and oxygen, showcasing both oxidation and reduction processes. Ozone (O₃) can also disproportionate, typically leading to the formation of oxygen oxide species.

Thus, ClO is the substance that does not exhibit disproportionation, making it the correct answer in this context.