Which of the following best describes an oxidising agent?

An oxidizing agent is defined as a substance that facilitates oxidation by accepting electrons from another species, thereby undergoing reduction itself. In the context of redox reactions, oxidation refers to the loss of electrons, while reduction is the gain of electrons. Therefore, when a substance acts as an oxidizing agent, it gains electrons, resulting in the reduction of its oxidation state.

Thus, when considering the provided description, the choice that accurately encompasses this definition highlights the role of an oxidizing agent in a chemical reaction. It clarifies that the substance not only assists in the process of oxidation occurring elsewhere but also undergoes a transformation of its own properties in doing so.

The other options do not accurately capture the essence of an oxidizing agent. For instance, a substance that allows reduction to happen by losing electrons describes a reducing agent rather than an oxidizing one. Similarly, while substances that do not change during a reaction can be catalysts or inert participants, they do not serve the defining role of an oxidizing agent. Lastly, conductivity in molten form relates to ionic movement rather than the definition or behavior of oxidizing agents regarding electron transfer and oxidation-reduction processes.