Which of the following substances is an oxidizing agent?

An oxidizing agent is defined as a substance that accepts electrons during a chemical reaction. This process facilitates the oxidation of another substance, which is losing electrons. When the oxidizing agent accepts electrons, it itself undergoes reduction (the gain of electrons). This key feature allows us to identify the correct answer clearly.

In the context of redox reactions, the interaction between the oxidizing agent and the reducing agent (the substance that donates electrons) is essential for the overall electron transfer process. The oxidizing agent is crucial for enabling this change, as it effectively "pulls" electrons away from the reducing agent. Thus, identifying the oxidizing agent as one that accepts electrons illustrates its role in promoting reduction and oxidation simultaneously.

The other choices relate to different characteristics that do not align with the definition of an oxidizing agent. For instance, a substance that donates electrons refers to a reducing agent, while a substance that causes reduction is inherently involved in the reduction process but does not directly define the property of accepting electrons. Similarly, a substance that remains unchanged during a reaction does not accurately reflect an oxidizing agent's active role in the electron transfer process.