Final answer:
In this reaction, zinc is being oxidized and sulfur is the oxidizing agent. Sulfur is causing zinc to be oxidized by accepting electrons. Zinc is the reducing agent as it causes sulfur to gain electrons and become reduced.
Step-by-step explanation:
In the reaction above, zinc is being oxidized by losing electrons. However, there must be another substance present that gains those electrons and in this case that is the sulfur. In other words, the sulfur is causing the zinc to be oxidized. Sulfur is called the oxidizing agent. The zinc causes the sulfur to gain electrons and become reduced and so the zinc is called the reducing agent.
The oxidizing agent is a substance that causes oxidation by accepting electrons; therefore, its oxidation state decreases. The reducing agent is a substance that causes reduction by losing electrons; therefore its oxidation state increases. The simplest way to think of this is that the oxidizing agent is the substance that is reduced, while the reducing agent is the substance that is oxidized.