In chemical reactions, what is the characteristic of an oxidising agent?

An oxidising agent is defined by its ability to facilitate the oxidation process by accepting electrons from another substance. This process involves the oxidising agent undergoing reduction, as it gains electrons while the other substance loses them, thus becoming oxidised. The act of removing electrons from other substances is fundamental to understanding oxidation and reduction reactions, which are key components of many chemical processes.

To clarify the roles involved, in redox reactions, oxidation refers to the loss of electrons, and reduction is the gain of electrons. The oxidising agent is crucially involved in these half-reactions by effectively "stealing" electrons, which is how it promotes the oxidation of another substance.

The other choices listed do not accurately describe the behavior of oxidising agents. Donating protons or losing/gaining protons pertains more to acid-base reactions rather than to oxidation-reduction mechanisms. Thus, the defining characteristic of an oxidising agent lies in its propensity to remove electrons from other species, confirming the correctness of the answer given.