Which among the following is the strongest oxidising agent?

The correct option is O₃.

An oxidising agent is a substance that has the ability to oxidise other substances by accepting electrons from them, thereby itself getting reduced. The strength of an oxidising agent depends on its tendency to gain electrons (its reduction potential) and its thermodynamic stability. Among the given options, Ozone ($O_3$) is the strongest oxidising agent due to its thermodynamic instability and its tendency to readily decompose to release highly reactive nascent oxygen.

Let's look at the chemical behavior of ozone. Ozone decompose readily to form diatomic oxygen and a nascent oxygen atom according to the following reaction:
$O_3\Rightarrow O_2+\; O$ (nascent oxygen)
This nascent oxygen is extremely reactive and readily abstracts electrons or combines with other substances, making ozone a exceptionally powerful oxidising agent, stronger than hydrogen peroxide ($H_2{O}_2$), potassium permanganate ($K\mathrm{Mn}{O}_4$), and potassium dichromate ($K_2{\mathrm{Cr}}_2{O}_7$).